ice: Add helper function to get the VF's VSI

Currently, the driver gets the VF's VSI by using a long string of
dereferences (i.e. vf->pf->vsi[vf->lan_vsi_idx]). If the method to get
the VF's VSI were to change the driver would have to change it in every
location. Fix this by adding the helper ice_get_vf_vsi().

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>