igb: increase timeout for ethtool offline self-test

On several machines with i350 adapters the ethtool offline self-test sometimes
fails. This happens because link auto negotiation may take longer than the
timeout of 4 seconds. Increasing the timeout by 1 seconds resolves the issue.

Output from a failing i350 offline self-test:
while [ 1 ]; do ethtool -t eth2 offline; done
The test result is PASS
The test extra info:
Register test  (offline)         0
Eeprom test    (offline)         0
Interrupt test (offline)         0
Loopback test  (offline)         0
Link test   (on/offline)         0

The test result is FAIL
The test extra info:
Register test  (offline)         0
Eeprom test    (offline)         0
Interrupt test (offline)         0
Loopback test  (offline)         0
Link test   (on/offline)         1

The test result is PASS
The test extra info:
Register test  (offline)         0
Eeprom test    (offline)         0
Interrupt test (offline)         0
Loopback test  (offline)         0
Link test   (on/offline)         0
Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igb/igb_ethtool.c

@@ -1891,7 +1891,7 @@ static int igb_link_test(struct igb_adapter *adapter, u64 *data)
 	} else {
 		hw->mac.ops.check_for_link(&adapter->hw);
 		if (hw->mac.autoneg)
-			msleep(4000);
+			msleep(5000);
 
 		if (!(rd32(E1000_STATUS) & E1000_STATUS_LU))
 			*data = 1;