rt2x00: fix beaconing on USB

When disable beaconing we clear register with beacon and newer set it
back, what make we stop send beacons infinitely.

Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>