Fix panic trying to use monitor mode. The iwn_cmd() calls issued by

iwn_config() want to msleep() on the mutex.

PR:		kern/138427
Submitted by:	Henry Hu <henry.hu.sh at gmail.com>
MFC after:	3 days

sys/dev/wpi/if_wpi.c

@@ -3561,7 +3561,9 @@ wpi_set_channel(struct ieee80211com *ic)
 	 * are already taken care of by their respective firmware commands.
 	 */
 	if (ic->ic_opmode == IEEE80211_M_MONITOR) {
+		WPI_LOCK(sc);
 		error = wpi_config(sc);
+		WPI_UNLOCK(sc);
 		if (error != 0)
 			device_printf(sc->sc_dev,
 			    "error %d settting channel\n", error);