wl1251: 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.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Vidhya Govindan <vidhya.govindan@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>