net: phy: replace mutex_is_locked with lockdep_assert_held in phylib

Switch to lockdep_assert_held(_once), similar to what is being done
in other subsystems. One advantage is that there's zero runtime
overhead if lockdep support isn't enabled.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/ccc40b9d-8ee0-43a1-5009-2cc95ca79c85@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>