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: 7efa84b7abc1 ('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>

drivers/net/ethernet/intel/i40e/i40e_virtchnl_pf.c

@@ -1774,7 +1774,7 @@ int i40e_vc_process_vf_msg(struct i40e_pf *pf, u16 vf_id, u32 v_opcode,
 			   u32 v_retval, u8 *msg, u16 msglen)
 {
 	struct i40e_hw *hw = &pf->hw;
-	int local_vf_id = vf_id - hw->func_caps.vf_base_id;
+	unsigned int local_vf_id = vf_id - hw->func_caps.vf_base_id;
 	struct i40e_vf *vf;
 	int ret;