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>