ice: Fix VSI rebuild WARN_ON check for VF

In commit b03d519d ("ice: store VF pointer instead of VF ID")
WARN_ON checks were added to validate the vsi->vf pointer and
catch programming errors. However, one check to vsi->vf was missed.
This caused a call trace when resetting VFs.

Fix ice_vsi_rebuild by encompassing VF pointer in WARN_ON check.

Fixes: b03d519d

 ("ice: store VF pointer instead of VF ID")
Signed-off-by: Benjamin Mikailenko <benjamin.mikailenko@intel.com>
Tested-by: Marek Szlosek <marek.szlosek@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>