wl12xx: remove unneeded locking

With the scan callback now being callable from
any context, these unlocks/locks can go away.
This makes the code easier to understand, since
callers of these functions must no longer be
aware that the mutex may be dropped.

As Stanislaw is working on iwlwifi scanning, I
didn't change it to take advantage of the new
mac80211 semantics.

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