net/mlx5e: Tunnel encap ETH header helper function

In tunnel encap we prepare the encap header for IPv4/6 cases, in two
separate functions. For ETH header generation the code is almost
duplicated.

Move the ETH header generation code from IPv4/6 functions to a helper
function, with no functional change.

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