drivers: staging: rtl8723au: fix "warning: cast to restricted __le16"

This patch fixes the sparse warning: "cast to restricted __le16" reported
for rtl8723au/hal/rtl8723au_xmit.c

Signed-off-by: Piotr Witoslawski <pwitos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>