libata: clear PIO pad area

ata_sff_data_xfer[32]() use pad area if the transfer size isn't
multiple of transfer size; however, this area wasn't cleared and
garbage data in pad area could be transferred to the device.  Make
sure the pad area is cleared.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Lei Ming <tom.leiming@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>