libata: skip 0xff polling for PATA controllers

In a presentation of true workmanship, pata_ali asserts IRQ
permanantly if the TF status register is read more than once when
there's no device attached to the port.

Avoid waiting polling for !0xff if it's PATA.  It's needed only for
some rare SATA devices anyway.

This problem is reported by Luca Tettamanti in bugzilla bug 9298.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Tested-By: Luca Tettamanti <kronos.it@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>