staging: r8712u: Fix Sparse endian warning in rtl871x_recv.c

Sparse reports the following:
  CHECK   drivers/staging/rtl8712/rtl871x_recv.c
drivers/staging/rtl8712/rtl871x_recv.c:657:21: warning: incorrect type in assignment (different base types)
drivers/staging/rtl8712/rtl871x_recv.c:657:21:    expected unsigned short [unsigned] [assigned] [usertype] len
drivers/staging/rtl8712/rtl871x_recv.c:657:21:    got restricted __be16 [usertype] <noident>

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>