net/mlx5e: Fix inline header size for small packets

[ Upstream commit 6aace17e ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>