staging: wfx: fix missing 'static' keyword

Sparse tool noticed that wfx_enable_beacon() is never used outside of
sta.c. Therefore, it can be declared static.
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200505123757.39506-11-Jerome.Pouiller@silabs.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/sta.c

@@ -519,7 +519,7 @@ void wfx_leave_ibss(struct ieee80211_hw *hw, struct ieee80211_vif *vif)
 	wfx_do_unjoin(wvif);
 }
 
-void wfx_enable_beacon(struct wfx_vif *wvif, bool enable)
+static void wfx_enable_beacon(struct wfx_vif *wvif, bool enable)
 {
 	// Driver has Content After DTIM Beacon in queue. Driver is waiting for
 	// a signal from the firmware. Since we are going to stop to send