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: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>