i40e: Fix firmware LLDP agent related warning

Make warning meaningful for the user.

Previously the trace:
"Starting FW LLDP agent failed: error: I40E_ERR_ADMIN_QUEUE_ERROR, I40E_AQ_RC_EAGAIN"
was produced when user tried to start Firmware LLDP agent,
just after it was stopped with sequence:
ethtool --set-priv-flags <dev> disable-fw-lldp on
ethtool --set-priv-flags <dev> disable-fw-lldp off
(without any delay between the commands)
At that point the firmware is still processing stop command, the behavior
is expected.

Fixes: c1041d0 ("i40e: Missing response checks in driver when starting/stopping FW LLDP")
Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Tested-by: Imam Hassan Reza Biswas <imam.hassan.reza.biswas@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/i40e/i40e_ethtool.c

@@ -5294,6 +5294,10 @@ static int i40e_set_priv_flags(struct net_device *dev, u32 flags)
 					dev_warn(&pf->pdev->dev,
 						 "Device configuration forbids SW from starting the LLDP agent.\n");
 					return -EINVAL;
+				case I40E_AQ_RC_EAGAIN:
+					dev_warn(&pf->pdev->dev,
+						 "Stop FW LLDP agent command is still being processed, please try again in a second.\n");
+					return -EBUSY;
 				default:
 					dev_warn(&pf->pdev->dev,
 						 "Starting FW LLDP agent failed: error: %s, %s\n",