net/mlx5: Enable SW-defined RoCEv2 UDP source port

When this is enabled, UDP source port for RoCEv2 packets are defined
by software instead of firmware.

Signed-off-by: Mark Zhang <markz@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>