ATA panic PR

Hello, everyone!

I have submitted a PR today.
Thought I should CC it here.

ATA panics immediately on boot if running in DMA mode (which is default)
With hw.ata.ata_dma="0" in /boot/loader.conf everything works fine, but
is terribly slow. DMA worked fine with 5.2.1-RELEASE.

# uname -a
FreeBSD urri.intercaf.ru 5.2-CURRENT FreeBSD 5.2-CURRENT #1: Wed Jun 30
10:36:24 MSD 2004     urri_at_urri.intercaf.ru:/usr/obj/usr/src/sys/URRI  i386

It panics with GENERIC kernel too.

atapci0: <SiS 961 UDMA100 controller> port
 0x110b-0x111a,0x376,0x170-0x177,0x3f6,0x1f0-0x1f7 at device 2.5 on pci0
ata0: at 0x1f0 irq 14 on atapci0
ata0-master: DMA limited to UDMA33, non-ATA66 cable or device
ad0: 19077MB <IC25N020ATMR04-0> [38760/16/63] at ata0-master UDMA33
ad0: TIMEOUT - READ_DMA retrying (2 retries left) LBA=39070017
ad0: DMA limited to UDMA33, non-ATA66 cable or device
kernel trap 12 with interrupts disabled

Fatal trap 12: page fault while in kernel mode
fault virtual address        = 0x74
fault code 			   = supervisor read, page not present
instruction pointer          = 0x8:0xc050f615
stack pointer                 = 0x10:0xc7683c48
frame pointer                 = 0x10:0xc7683c4c
code segment               = base 0x0, limit 0xfffff, type 0x1b
                                  = DPL 0, pres 1, def32 1, gran 1
processor eflags           = resume, IOPL = 0
current process            = 5 (taskqueue)
trap number                 = 12
panic: page fault

syncing disks, buffers remaining...
done
Uptime: 6s





Cheers,
          AL.