wl1251: Set the correct dtim period to the firmware

This patch sets the dtim period obtained from the mac80211 to firmware.

Signed-off-by: Vidhya Govindan <vidhya.govindan@nokia.com>
Reviewed-by: Janne Ylalehto <janne.ylalehto@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>