net: mvneta: remove unnecessary if condition in mvneta_xdp_submit_frame

Get rid of unnecessary if check on tx_desc pointer in
mvneta_xdp_submit_frame routine since num_frames is always greater than
0 and tx_desc pointer is always initialized.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>