rtl8xxxu: Fix non static symbol warning

Fixes the following sparse warning:

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_8192e.c:1559:6: warning:
 symbol 'rtl8192eu_power_off' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NJes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_8192e.c

@@ -1556,7 +1556,7 @@ static int rtl8192eu_power_on(struct rtl8xxxu_priv *priv)
 	return ret;
 }
 
-void rtl8192eu_power_off(struct rtl8xxxu_priv *priv)
+static void rtl8192eu_power_off(struct rtl8xxxu_priv *priv)
 {
 	u8 val8;
 	u16 val16;