pata-rb532-cf: use ata_sff_exec_command()

The only difference between rb532_pata_exec_command() and
ata_sff_exec_command() is added debugging output, so it can be dropped
and the standard op used instead.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>