SATA port multipliers

With this change (and other previously reported & PR'd patches) SATA
port multipliers seem to be working for me, albeit half as fast as expected.

The ata_identify() call was commented out in February apparently due to
other problems it caused but it appears to be the only thing that scans
for drives behind an enclosure.

Index: sys/dev/ata/ata-all.c
===================================================================
--- sys/dev/ata/ata-all.c    (revision 192136)
+++ sys/dev/ata/ata-all.c    (working copy)
_at__at_ -291,7 +291,7 _at__at_
     ATA_LOCKING(dev, ATA_LF_UNLOCK);
 
     /* Add new children. */
-/*    ata_identify(dev); */
+    ata_identify(dev);
 
     if (bootverbose)
     device_printf(dev, "reinit done ..\n");