Continuous timeout messages from 10.0 alpha4

Resending to current

This is a regression between alpha2 and alpha4. I first setup a disk to
boot 10 just after alpha2 was tagged. A few days ago I updated to
alpha4 and started getting timeout messages, I have just built alpha5
r256098 and still get them.

I just had 10 running for a few hours and 3 of the messages log archives
which syslog rotates each hour due to size contain 3370 2508 and 3162
timeout messages over 3 hours uptime.

each entry is similar to --
Oct  7 19:00:02 leader kernel: ahcich1: Timeout on slot 15 port 0
Oct  7 19:00:02 leader kernel: ahcich1: is 40000001 cs 00008000 ss 
00000000 rs 00008000 tfd 2451 serr 00000000 cmd 00004f17

With slot x cycling from 0-31 but not always in sequence.

MB is ASUS P8H61M LE/USB3 - corei5 - 8MB - nvidia GT520

ahci0: <Intel Cougar Point AHCI SATA controller> port 
0xf070-0xf077,0xf060-0xf063,0xf050-0xf057,0xf040-0xf043,0xf020-0xf03f 
mem 0xfb305000-0xfb3057ff irq 20 at device 31.2 on pci0
ahci0: AHCI v1.30 with 4 3Gbps ports, Port Multiplier not supported

With the following connected to ahcich1 (LG blu-ray burner)
cd0 at ahcich1 bus 0 scbus1 target 0 lun 0
cd0: <HL-DT-ST BD-RE  BH12LS38 1.00> Removable CD-ROM SCSI-0 device



Most relevant dmesg info would be (full dmesg/logs available) --

ahci0: <Intel Cougar Point AHCI SATA controller> port 
0xf070-0xf077,0xf060-0xf063,0xf050-0xf057,0xf040-0xf043,0xf020-0xf03f 
mem 0xfb305000-0xfb3057ff irq 20 at device 31.2 on pci0
ahci0: AHCI v1.30 with 4 3Gbps ports, Port Multiplier not supported
ahcich0: <AHCI channel> at channel 0 on ahci0
ahcich1: <AHCI channel> at channel 1 on ahci0
ahcich4: <AHCI channel> at channel 4 on ahci0
ahcich5: <AHCI channel> at channel 5 on ahci0
ahciem0: <AHCI enclosure management bridge> on ahci0

ada0 at ahcich0 bus 0 scbus0 target 0 lun 0
ada0: <WDC WD20EARX-00PASB0 51.0AB51> ATA-8 SATA 3.x device
ada0: 300.000MB/s transfers (SATA 2.x, UDMA6, PIO 8192bytes)
ada0: Command Queueing enabled
ada0: 1907729MB (3907029168 512 byte sectors: 16H 63S/T 16383C)
ada0: quirks=0x1<4K>
ada0: Previously was known as ad4
ada1 at ahcich4 bus 0 scbus2 target 0 lun 0
ada1: <WDC WD10EARS-00Y5B1 80.00A80> ATA-8 SATA 2.x device
ada1: 300.000MB/s transfers (SATA 2.x, UDMA6, PIO 8192bytes)
ada1: Command Queueing enabled
ada1: 953869MB (1953525168 512 byte sectors: 16H 63S/T 16383C)
ada1: quirks=0x1<4K>
ada1: Previously was known as ad8
ada2 at ahcich5 bus 0 scbus3 target 0 lun 0
ada2: <eSATA-2 WD800JD-23JNA1 06.01C06> ATA-6 SATA 2.x device
ada2: 150.000MB/s transfers (SATA 1.x, UDMA6, PIO 512bytes)
ada2: 76324MB (156312576 512 byte sectors: 16H 63S/T 16383C)
ada2: Previously was known as ad10
ses0 at ahciem0 bus 0 scbus4 target 0 lun 0
ses0: <AHCI SGPIO Enclosure 1.00 0001> SEMB S-E-S 2.00 device
ses0: SEMB SES Device