cfg80211: sched_scan_mtx lock in cfg80211_conn_work()

Introduced in f9f47529
("cfg80211: always check for scan end on P2P device")

cfg80211_conn_scan() which requires sched_scan_mtx to be held can be called
from cfg80211_conn_work(). Without this we are hitting multiple warnings like
the following:

WARNING: at net/wireless/sme.c:88 cfg80211_conn_scan+0x1dc/0x3a0 [cfg80211]()
Hardware name: 0578A21
Modules linked in: ...
Pid: 620, comm: kworker/3:1 Not tainted 3.9.0-rc4-next-20130328+ #326
Call Trace:
 [<c1036992>] warn_slowpath_common+0x72/0xa0
 [<c10369e2>] warn_slowpath_null+0x22/0x30
 [<faa4b0ec>] cfg80211_conn_scan+0x1dc/0x3a0 [cfg80211]
 [<faa4b344>] cfg80211_conn_do_work+0x94/0x380 [cfg80211]
 [<faa4c3b2>] cfg80211_conn_work+0xa2/0x130 [cfg80211]
 [<c1051858>] process_one_work+0x198/0x450

Signed-off-by: Artem Savkov <artem.savkov@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>