phy_init() and phy_exit() will have to do more stuff under rtnl_lock()
in a future change. Since rtnl_unlock() -> netdev_run_todo() does a lot
of stuff under the hood, it's a pity to lock and unlock the rtnetlink
mutex twice in a row.

Change the calling convention such that the only caller of
ethtool_set_ethtool_phy_ops(), phy_device.c, provides a context where
the rtnl_mutex is already acquired.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20230801142824.1772134-11-vladimir.oltean@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>