ice: count representor stats

Removing control plane VSI result in no information about slow-path
statistic. In current solution statistics need to be counted in driver.

Patch is based on similar implementation done by Simon Horman in nfp:
commit eadfa4c3 ("nfp: add stats and xmit helpers for representors")

Add const modifier to netdev parameter in ice_netdev_to_repr(). It isn't
(and shouldn't be) modified in the function.

Reviewed-by: Marcin Szycik <marcin.szycik@linux.intel.com>
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Tested-by: Sujai Buvaneswaran <sujai.buvaneswaran@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>