i40e: potential array underflow in i40e_vc_process_vf_msg()

If "vf_id" is smaller than hw->func_caps.vf_base_id then it leads to
an array underflow of the pf->vf[] array.  This is unlikely to happen
unless the hardware is bad, but it's a small change and it silences a
static checker warning.

Fixes: 7efa84b7 ('i40e: support VFs on PFs other than 0')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>