[libata] ata_piix: cleanup dmi strings checking

Commit
	ATA: piix, fix pointer deref on suspend
fixed a possible oops in an ugly manner. Use newly introduced dmi_match()
to make the code pretty again.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Alexandru Romanescu <a_romanescu@yahoo.co.uk>
Cc: Tejun Heo <tj@kernel.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>