rt2x00: change order when stop beaconing

When no beaconing is needed, first stop beacon queue (disable beaconing
globally) to avoid possible sending of not prepared beacon on short
period after clearing beacon and before stop of BCN queue.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>