net/mlx5e: Use helpers to get headers criteria and value pointers

The headers criteria and value pointers may be either of the inner
packet, if a tunnel exists, or of the outer. Simplify the code by using
helper functions to retrieve them.

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