net/mlx5e: Fix null pointer access when setting MTU of vport representor

MTU helper function is used by both conventional mlx5e
instances (PF/VF) and the eswitch representors. The representor
shouldn't change the nic vport context MTU, the VF is responsible for
that. Therefore set_mtu_cb has a null value when changing the
representor MTU.

Fixes: 250a42b6 ("net/mlx5e: Support configurable MTU for vport representors")
Signed-off-by: Adi Nissim <adin@mellanox.com>
Reviewed-by: Yevgeny Kliteynik <kliteyn@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

drivers/net/ethernet/mellanox/mlx5/core/en_main.c

@@ -3712,7 +3712,8 @@ int mlx5e_change_mtu(struct net_device *netdev, int new_mtu,
 
 	if (!reset) {
 		params->sw_mtu = new_mtu;
-		set_mtu_cb(priv);
+		if (set_mtu_cb)
+			set_mtu_cb(priv);
 		netdev->mtu = params->sw_mtu;
 		goto out;
 	}