Check bit 2 of Word 53 for Word 88 validity before using Word 88 to
determine UDMA mask.  Note that the original xfer mask implementation
using ata_get_mode_mask() didn't consider bit 2 of Word 53.  This
patch introduces different (correct) behavior.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>