staging: rtl8723au: process_80211d(): Fix order of advancing array pos

Fix smatch warning from advancing array index before reading out the
value of the array.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>