net/mlx5e: Tunnel routing output devs helper function

For tunnel we determine the output devs for IPv4/6 cases, in two
separate functions, with a duplicated code.

Move that 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>