Move ieee80211_scan_cancel() and all other related code to
ieee80211_restart_work() as ieee80211_restart_hw() is intended to be
callable from any context.

Fix a bug that RTNL lock is not taken during ieee80211_cancel_scan().

Take local->mtx before WARN(test_bit(SCAN_HW_SCANNING, &local->scanning)
to prevent the race condition with __ieee80211_start_scan() described
here: http://marc.info/?l=linux-wireless&m=128516716810537&w=2Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>