rt2x00: handle spurious pci interrupts

We have documented case of very bad performance issue on rt2800pci
device, because it generate spurious interrupt, what cause irq line
is disabled: https://bugzilla.redhat.com/show_bug.cgi?id=658451

We already address that problem in separate patch by returning
IRQ_HANDLED from interrupt handler. We think similar fix is needed for
other rt2x00 PCI devices, because users report performance problems on
these devices too.

Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>