[PATCH] libata: fix ata_xfer_tbl termination

ata_xfer_tbl is terminated by entry with -1 as ->shift.  However,
->shift was unsigned int making the termination condition bogus.  This
patch converts ->shift and ->bits to int.

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