ethtool: use phydev variable

In ethtool_get_phy_stats(), the phydev varaible is set to
dev->phydev but dev->phydev is still used.  Replace
dev->phydev uses with phydev.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>