wl12xx: Fix potential interrupt storm

The interrupt threaded handler exits immediately if the driver's state
is WL1271_STATE_OFF. As a result, the interrupt status is not read. If
the interrupt is level triggered, it will be fired again.

Fix this by disabling interrupts before setting the state to OFF.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>