libertas: fix non static symbol warning

Fixes the following sparse warning:

drivers/net/wireless/marvell/libertas/cfg.c:2047:5: warning:
 symbol 'lbs_set_power_mgmt' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/marvell/libertas/cfg.c

@@ -2044,8 +2044,8 @@ static int lbs_leave_ibss(struct wiphy *wiphy, struct net_device *dev)
 
 
 
-int lbs_set_power_mgmt(struct wiphy *wiphy, struct net_device *dev,
-		       bool enabled, int timeout)
+static int lbs_set_power_mgmt(struct wiphy *wiphy, struct net_device *dev,
+			      bool enabled, int timeout)
 {
 	struct lbs_private *priv = wiphy_priv(wiphy);