ice: make representor code generic

Representor code needs to be independent from specific device type, like
in this case VF. Make generic add / remove representor function and
specific add VF / rem VF function. New device types will follow this
scheme.

In bridge offload code there is a need to get representor pointer based
on VSI. Implement helper function to achieve that.

Reviewed-by: Piotr Raczynski <piotr.raczynski@intel.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@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>