net: driver: stmicro: Remove some useless the lock protection

kernel always invokes a pair of rtnl_lock adn rtnl_unlock to
protect dev_ethtool(), so its not neccessary to invoke spin_lock/unlock
in ethtool_ops.

Signed-off-by: Yang Wei <Wei.Yang@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>