ice: introduce common helper for retrieving VSI by vsi_num

Both ice_idc.c and ice_virtchnl.c carry their own implementation of a
helper function that is looking for a given VSI based on provided
vsi_num. Their functionality is the same, so let's introduce the common
function in ice.h that both of the mentioned sites will use.

This is a strictly cleanup thing, no functionality is changed.

Reviewed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>