wl1271: mask aid bits 14 and 15 in ps-poll template

In ps-poll template aid bits 14 and 15 were not masked as required by
the standard. Mask them so that aid is sent in correct format.

This patch is a direct port of the respective patch for the wl1251
driver by Kalle Valo.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Vidhya Govindan <vidhya.govindan@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>