ata_if_xfermask() word 51 fix

If word 53 bit 1 isn't set, the maximum PIO mode is indicated by
the upper 8 bits of word 51, not the lower 8 bits.  Fixes PIO mode
detection on old Compact Flash cards.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>