wcn36xx: enable the beaconing in mesh mode

Enable the beaconing in wnc36xx by tweaking the tim offset and
force the use of AP-style beaconing. Otherwise, beaconing is not
working. The tim offset is set to 256. Otherwise, this will
overwrite mesh beacon submitted by mac80211.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>