[PATCH] libata: check Word 88 validity in ata_id_xfer_mask()

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