We should not write 0 to the beacon sync register during
config_intf() since that will clear out the beacon interval
and forces the beacon to be send out at the lowest interval.
(reported by Mattias Nissler).

The side effect of the same bug was that while working with
multiple virtual AP interfaces a change for any of those
interfaces would disable beaconing untill an beacon update
was provided.

This is resolved by only updating the TSF_SYNC value during
config_intf(). In update_beacon() we disable beaconing
temporarily to prevent fake beacons to be transmitted.
Finally kick_tx_queue() will enable beaconing again.
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>