net: stmmac: drop the ethtool begin() callback

This callback doesn't seem to serve much purpose, and prevents things
like:

    - systemd.link files from disabling autonegotiation
    - carrier detection in NetworkManager
    - any ethtool setting

prior to userspace bringing the link up.

The only fear I can think of is accessing unclocked resources due to
pm_runtime, but ethtool ioctls handle that as of commit
f32a2137 ("ethtool: runtime-resume netdev parent before ethtool ioctl ops")

Reviewed-by: Dmitry Dolenko <d.dolenko@metrotek.ru>
Tested-by: Dmitry Dolenko <d.dolenko@metrotek.ru>
Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>