igc: Introduce TX/RX stats helpers

In preparation for AF_XDP zero-copy support, encapsulate the code that
updates the driver RX stats in its own local helper so it can be reused
in the zero-copy path. Likewise, encapsulate TX stats code as well.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Signed-off-by: Vedang Patel <vedang.patel@intel.com>
Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>