pata_legacy: fix a couple uninitialized variable bugs

commit 01392347 upstream.

The last byte of "pad" is used without being initialized.

Fixes: 55dba312 ("libata: update ->data_xfer hook for ATAPI")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>