rt2x00pci: Use PCI MSIs whenever possible

All PCIe devices must support MSIs, make use of them.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>