[PATCH] libata: consider disabled devices in ata_dev_xfermask()

ata_bus_probe() now marks failed devices properly and leaves
meaningful transfer mode masks.  This patch makes ata_dev_xfermask()
consider disable devices when determining PIO mode to avoid violating
device selection timing.

While at it, move port-wide resttriction out of device iteration loop
and try to make the function look a bit prettier.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>