net/mlx5e: IPoIB, Set the netdevice sw mtu in ipoib enhanced flow

After introduction of the cited commit, mlx5e_build_nic_params
receives the netdevice mtu in order to set the sw_mtu of mlx5e_params.
For enhanced IPoIB, the netdevice mtu is not set in this stage,
therefore, the initial sw_mtu equals zero. As a result, the hw_mtu
of the receive queue will be calculated incorrectly causing traffic
issues.

To fix this issue, query for port mtu before building the nic params.

Fixes: 472a1e44 ("net/mlx5e: Save MTU in channels params")
Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>