rt2x00: Fix beacon de-synchronization while update beacon

When beacon is being updated to refresh TIM (AP mode), beacon frames are
de-synchronizing (i.e. two neighbor beacon frames - before and after
update - are being transmitted with a wrong time interval).
That is because xxx_write_beacon should disable beacon generation only
while beacon data are being uploaded to the device, but it should not
disable the beacon clock.

Signed-off-by: Igor Perminov <igor.perminov@inbox.ru>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>