mac80211: make conf_tx non-atomic

The conf_tx callback currently needs to be atomic, this requirement
is just because it can be called from scanning. This rearranges it
slightly to only update while not scanning (which is fine, we'll be
getting beacons when associated) and thus removes the atomic
requirement.

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