rtlwifi: rtl8192cu: Fix sparse non static symbol warning

Fixes the following sparse warning:

drivers/net/wireless/rtlwifi/rtl8192cu/hw.c:1595:6: warning:
 symbol 'usb_cmd_send_packet' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rtlwifi/rtl8192cu/hw.c

@@ -1592,7 +1592,7 @@ void rtl92cu_get_hw_reg(struct ieee80211_hw *hw, u8 variable, u8 *val)
 	}
 }
 
-bool usb_cmd_send_packet(struct ieee80211_hw *hw, struct sk_buff *skb)
+static bool usb_cmd_send_packet(struct ieee80211_hw *hw, struct sk_buff *skb)
 {
   /* Currently nothing happens here.
    * Traffic stops after some seconds in WPA2 802.11n mode.