ata_piix: remove open-coded dmi_match(DMI_OEM_STRING)

Since de40614d

 ("firmware: dmi_scan: Add DMI_OEM_STRING support to
dmi_matches") dmi_check_system() can match OEM_STRINGs itself.
Use the feature.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Jens Axboe <axboe@kernel.dk>