i40e/i40evf: Fix compile issue related to const string

Add const to functions that return strings that aren't going to be
modified. This addresses some reported compile complaints.

Change-ID: Ic56b1e814ab4d23a50480e7fdec652445f776ee8
Signed-off-by: Jingjing Wu <jingjing.wu@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
diff --git a/drivers/net/ethernet/intel/i40evf/i40e_common.c b/drivers/net/ethernet/intel/i40evf/i40e_common.c
index b98b642..7435fb3 100644
--- a/drivers/net/ethernet/intel/i40evf/i40e_common.c
+++ b/drivers/net/ethernet/intel/i40evf/i40e_common.c
@@ -87,7 +87,7 @@
  * @hw: pointer to the HW structure
  * @aq_err: the AQ error code to convert
  **/
-char *i40evf_aq_str(struct i40e_hw *hw, enum i40e_admin_queue_err aq_err)
+const char *i40evf_aq_str(struct i40e_hw *hw, enum i40e_admin_queue_err aq_err)
 {
 	switch (aq_err) {
 	case I40E_AQ_RC_OK:
@@ -147,7 +147,7 @@
  * @hw: pointer to the HW structure
  * @stat_err: the status error code to convert
  **/
-char *i40evf_stat_str(struct i40e_hw *hw, i40e_status stat_err)
+const char *i40evf_stat_str(struct i40e_hw *hw, i40e_status stat_err)
 {
 	switch (stat_err) {
 	case 0: