libata-sff: PCI IRQ handling fix

It is legitimate (although annoying and silly) for a PCI IDE controller
not to be assigned an interrupt and to be polled. The libata-sff code
should therefore not try and request IRQ 0 in this case.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>