libata: fix G5 SATA broken on -rc5

Fix G5 SATA irq 18: nobody cared, reported on -rc5 by Olaf Hering:
fixlet to a57c1bad libata-sff:
Fix oops reported in kerneloops.org for pnp devices with no ctl

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Tested-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>