net/mlx5e: Refactor VXLAN tunnel decap offloading code

Separates the vxlan header match handling from the matching on the
general fields of ipv4/6 tunnels, thus allowing the common IP tunnel
match code to branch in down stream patch, to multiple IP tunnels.

This patch doesn't add any functionality.

Signed-off-by: Oz Shlomo <ozsh@mellanox.com>
Reviewed-by: Eli Britstein <elibr@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>