i40e: Fix display statistics for veb_tc

If veb-stats was enabled, the ethtool stats triggered a warning
due to invalid size: 'unexpected stat size for veb.tc_%u_tx_packets'.
This was due to an incorrect structure definition for the statistics.
Structures and functions have been improved in line with requirements
for the presentation of statistics, in particular for the functions:
'i40e_add_ethtool_stats' and 'i40e_add_stat_strings'.

Fixes: 1510ae0b

 ("i40e: convert VEB TC stats to use an i40e_stats array")
Signed-off-by: Eryk Rybak <eryk.roch.rybak@intel.com>
Signed-off-by: Grzegorz Szczurek <grzegorzx.szczurek@intel.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>