libata: use dev_driver_string() instead of "libata" in libata-sff.c

libata-sff code used DRV_NAME which is hardcoded to "libata" when
requesting resources.  Use dev_driver_string() such that low level
driver names are used in resource listing.

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