rt2x00: Don't frequently reset beacon interval in AdHoc mode

rt2x00: Don't frequently reset beacon interval in AdHoc mode

Commit f64cff1514f911fe7435c18345756124c54d9a79 "Check for specific changed
flags when updating the erp config" changed the way in which a new beacon
interval gets handled. However, due to a bug in rt2800usb and rt2800pci the
beacon interval was reset during each scan, thus causing problems in AdHoc
mode.

Fix this by not cleaning up the beacon interval when killing the beacon queue
but just prevent the device from sending out beacons.

Reported-by: Wolfgang Kufner <wolfgang.kufner@gmail.com>
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>