ice: refactor VF control VSI interrupt handling

All VF control VSIs share the same interrupt vector. Currently, a helper
function dedicated for that directly sets ice_vsi::base_vector.

Use helper that returns pointer to first found VF control VSI instead.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Piotr Raczynski <piotr.raczynski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>