staging: rtl8723au: fix incorrect type in assignment warning

Repaced calls to htons and memcpy with a single call to put_unaligned_be16
to fix the following sparse warning:
drivers/staging/rtl8723au/core/rtw_recv.c:1557:21: warning: incorrect type in assignment (different base types)
drivers/staging/rtl8723au/core/rtw_recv.c:1557:21:    expected unsigned short [unsigned] [assigned] [usertype] len
drivers/staging/rtl8723au/core/rtw_recv.c:1557:21:    got restricted __be16 [usertype] <noident>

Signed-off-by: Steve Pennington <sgpenn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>