rt2x00: Introduce beacon_update_locked that requires caller locking

Introduce a beacon_update_locked function that does not acquire the
according beacon mutex to allow beacon updates from atomic context. The
caller has to take care of synchronization.

No functional changes. Just preparation for beacon updates from tasklet
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>