net/mlx5e: Helper function to set ethertype

Set ethertype match in a helper function as a pre-step towards
optimizing it.

Signed-off-by: Eli Britstein <elibr@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>