net/mlx4_en: Fix user prio field in XDP forward

The user prio field is wrong (and overflows) in the XDP forward
flow.
This is a result of a bad value for num_tx_rings_p_up, which should
account all XDP TX rings, as they operate for the same user prio.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reported-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>