stmmac: remove ethtool driver version info

I think there's no benefit in reporting a date from almost 6 yrs ago.
Let ethtool report the default (kernel version) instead.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>