wl1251: use beacon interval and dtim period provided by mac80211

wl1251 was using hardcoded beacon intervals and dtim periods, use the ones
provided by mac80211 instead.

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