wl1251: enable beacon early termination while in power-saving mode

Port the beacon early termination feature from wl1251 driver version
included in the Maemo Fremantle kernel.
It is enabled when going to power-saving mode and disabled when leaving
power-saving mode.

Signed-off-by: David Gnedt <david.gnedt@davizone.at>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>