mac80211: fix driver offchannel notification when the channel does not change

When running in client mode and associating to an AP, the channel
change is usually performed with the offchannel flag still set.
However after the assoc is complete, the following channel change event
is suppressed because the run time channel is already set to the operating channel.
Fix this by sending channel change notifications to the driver even if
only the offchannel flag changes.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>