rtl8xxxu: Fix sparse warning: cast from restricted __le16

Fix the warning reported by sparse as:
 drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:4819:17: sparse: sparse: cast from restricted __le16
 drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:4892:17: sparse: sparse: cast from restricted __le16

Signed-off-by: Chris Chiu <chiu@endlessm.com>
Reported-by: kbuild test robot <lkp@intel.com>
Acked-by: Jes Sorensen <jes@trained-monkey.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200319064341.49500-1-chiu@endlessm.com