wl1271: Don't mask interrupts while handling interrupt

Don't mask firmware interrupts while processing interrupts. This allows the
interrupt handler looping to work efficiently thus reducing interrupt
processing latency.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>