dbri: move dereference after check for NULL

We accidentally introduced a dereference before the NULL check in
xmit_descs() as part of silencing a GCC warning.

Fixes: 16f46050

 ("dbri: Fix compiler warning")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>