mac80211: config hw when going back on-channel

When going back on-channel, we should reconfigure
the hw iff the hardware is not already configured
to the operational channel.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Cc: stable@kernel.org # 2.6.39+
Signed-off-by: John W. Linville <linville@tuxdriver.com>