net: pcs: lynx: remove lynx_get_mdio_device()

lynx_get_mdio_device() is no longer necessary, let's remove it so the
lynx PCS code is always managing the lifetime of the mdiodev.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>