mac80211: do not monitor the connection while scanning

mac80211 constantly monitors the connection to the associated AP
in order to check if it is out of reach/dead.

This is absolutely fine most of the time.
Except when there is a scheduled scan for the whole neighborhood.
After all this path could trigger while scanning on different channel.
Or even worse: this AP probing triggers a WARN_ON in rate_lowest_index
when the scan code did a band transition!
( http://www.kerneloops.org/raw.php?rawid=449304

 )

Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Christian Lamparter <chunkeey@web.de>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>