Add description for each meta data. Redefine tunnel mask to match only
tunneled MAC and tunneled VLAN. It shouldn't try to match other flags
(previously it was 0xff, it is redundant).

VLAN mask was 0xd000, change it to 0xf000. 4 last bits are flags
depending on the same field in packets (VLAN tag). Because of that,
It isn't harmful to match also on ITAG.

Group all MDID and MDID offsets into enums to keep things organized.
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Reviewed-by: Piotr Raczynski <piotr.raczynski@intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Tested-by: Sujai Buvaneswaran <sujai.buvaneswaran@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>