ice: stop hard coding the ICE_VSI_CTRL location

When allocating the ICE_VSI_CTRL, the allocated struct ice_vsi pointer is
stored into the PF's pf->vsi array at a fixed location. This was
historically done on the basis that it could provide an O(1) lookup for the
special control VSI.

Since we store the ctrl_vsi_idx, we already have O(1) lookup regardless of
where in the array we store this VSI.

Simplify the logic in ice_vsi_alloc by using the same method of storing the
control VSI as other types of VSIs.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>