wl1271: Aggregate RX acknowledgements to FW

This patch will ack RX frames read from the firmware in one single write,
instead of acking all the frames separately. This will reduce the amount of
required communication per frame.

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>