net/mlx5e: Refactor mod header management API

For all mod hdr related functions to reside in a single self contained
component (mod_hdr.c), refactor alloc() and add get_id() so that user
won't rely on internal implementation, and move both to mod_hdr
component.

Rename the prefix to mlx5e_mod_hdr_* as other mod hdr functions.

Signed-off-by: Paul Blakey <paulb@nvidia.com>
Reviewed-by: Oz Shlomo <ozsh@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>