mac80211: disable beacon monitor while going offchannel

The beacon monitor should be disabled when going off channel
to prevent spurious warnings and triggering connection
deterioration work such as sending probe requests. Re-enable
the beacon monitor once we come back to the home channel.

This patch has fixes for stable kernels [2.6.34+].

Cc: stable@kernel.org
Cc: Paul Stewart <pstew@google.com>
Cc: Amod Bodas <amod.bodas@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>