net/mlx5e: Fix traffic between VF and representor

After the cited commit, WQE RQ size is calculated based on sw_mtu but it
was not set for representors. This commit fixes that.

Fixes: 472a1e44 ("net/mlx5e: Save MTU in channels params")
Signed-off-by: Shahar Klein <shahark@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>