net: phy: stop exporting phy_driver_register

phy_driver_register() isn't used outside phy_device.c any longer,
so we can stop exporting it.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/dff44b83-4a85-4fff-bf6b-f12efd97b56e@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>