staging: rtl8723au: fix sparse warning

change cast to __le16 to fix the following warning:
drivers/staging/rtl8723au/hal/rtl8723a_hal_init.c:1488:20: warning: cast to restricted __le16

Signed-off-by: Juston Li <juston.h.li@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>