net/mlx5e: Fix inline header size for small packets

Fix inline header size, make sure it is not greater than skb len.
This bug effects small packets, for example L2 packets with size < 18.

Fixes: ae76715d ("net/mlx5e: Check the minimum inline header mode before xmit")
Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>