mac80211: unify scan and work mutexes

Having both scan and work mutexes is not just
a bit too fine grained, it also creates issues
when there's code that needs both since they
then need to be acquired in the right order,
which can be hard to do.

Therefore, use just a single mutex for both.

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