mac80211: set channel back after disassociating

As we've discussed, we want to avoid channel changes
while associated. While the part when we actually
associate needs a bit more work, the bit that happens
on disassociating can be changed quite easily. Move
the channel type change later in the disassociate
process to set the channel only after the driver was
told that it's now disassociated.

As the driver could expect powersave to be enabled
only when associated, this thus results in splitting
the config call, but overall what happens makes more
sense this way.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>