rtl8xxxu: gen1: Fix non static symbol warning

Fixes the following sparse warning:

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:898:1: warning:
 symbol 'rtl8xxxu_gen1_h2c_cmd' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c

@@ -894,7 +894,7 @@ int rtl8xxxu_write_rfreg(struct rtl8xxxu_priv *priv,
 	return retval;
 }
 
-int
+static int
 rtl8xxxu_gen1_h2c_cmd(struct rtl8xxxu_priv *priv, struct h2c_cmd *h2c, int len)
 {
 	struct device *dev = &priv->udev->dev;