mac80211: Allow to set channel in adhoc properly

The last patch fixes a bug that it was not possible to set the channel
manually in the ad hoc mode properly.

Please commit this patches so that we don't need the proprietary
Broadcom driver in the near future anymore.

Signed-off-by: Alina Friedrichsen <x-alina@gmx.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>