rt2x00: fix beaconing on USB

commit 8834d3608cc516f13e2e510f4057c263f3d2ce42 upstream.

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

Change-Id: Ia2046e241ea0b5f91f6e1a56959d5e52b1ed8ffe
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>