wl1251: Increase the beacon loss timeout value and handle regain event

This patch increases the number of beacons to be missed before generating
SYNC TIMEOUT event. It increases the beacon timeout period to 500
microseconds, which gives enough time for the firmware to receive probe
response or beacon. Also handled the regain event from firmware once it
receives a probe response or beacon.

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>