Allow the beacondone and pretbtt functions to update the beacon from
atomic context by using the beacon update functions with caller locking.

This is a preparation for moving the periodic beacon handling into
tasklets that require atomic context.
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>