ixgbevf: ixgbevf_write/read_posted_mbx should use IXGBE_ERR_MBX to initialize ret_val

Now ixgbevf_write/read_posted_mbx use -IXGBE_ERR_MBX as the initiative
return value, but it's incorrect, cause in ixgbevf_vlan_rx_add_vid(),
it use err == IXGBE_ERR_MBX, the err returned from mac.ops.set_vfta,
and in ixgbevf_set_vfta_vf, it return from write/read_posted. so we
should initialize err with IXGBE_ERR_MBX, instead of -IXGBE_ERR_MBX.

With this fix, the other functions that called it also can work well,
cause they only care about if err is 0 or not.
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbevf/mbx.c

@@ -85,7 +85,7 @@ static s32 ixgbevf_poll_for_ack(struct ixgbe_hw *hw)
 static s32 ixgbevf_read_posted_mbx(struct ixgbe_hw *hw, u32 *msg, u16 size)
 {
 	struct ixgbe_mbx_info *mbx = &hw->mbx;
-	s32 ret_val = -IXGBE_ERR_MBX;
+	s32 ret_val = IXGBE_ERR_MBX;
 
 	if (!mbx->ops.read)
 		goto out;
@@ -111,7 +111,7 @@ static s32 ixgbevf_read_posted_mbx(struct ixgbe_hw *hw, u32 *msg, u16 size)
 static s32 ixgbevf_write_posted_mbx(struct ixgbe_hw *hw, u32 *msg, u16 size)
 {
 	struct ixgbe_mbx_info *mbx = &hw->mbx;
-	s32 ret_val = -IXGBE_ERR_MBX;
+	s32 ret_val = IXGBE_ERR_MBX;
 
 	/* exit if either we can't write or there isn't a defined timeout */
 	if (!mbx->ops.write || !mbx->timeout)