lan78xx: Fix boot-time RTNL warning

lan78xx_set_eee calls phylink_ethtool_set_eee, which WARNs if rtnl_lock
has not been called. This is fine when called via the ethtool_ops
.set_eee entry point, presumably because the lock is already held, but
lan78xx_probe also calls lan78xx_set_eee via lan78xx_phy_init, which
requires an explicit rtnl_lock/unlock pair to avoid the warning.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

drivers/net/usb/lan78xx.c

@@ -2935,7 +2935,9 @@ static int lan78xx_phy_init(struct lan78xx_net *dev)
 					 "microchip,tx-lpi-timer",
 					 &edata.tx_lpi_timer))
 			edata.tx_lpi_timer = 600; /* non-aggressive */
+		rtnl_lock();
 		(void)lan78xx_set_eee(dev->net, &edata);
+		rtnl_unlock();
 
 		phy_support_eee(phydev);
 	}