net: stmmac: rk: remove obsolete .set_*_speed() methods

Now that no SoC implements the .set_*_speed() methods, we can get rid
of these methods and the now unused code in rk_set_clk_tx_rate().
Arrange for the function to return an error when the .set_speed()
method is not implemented.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uPk3O-004CFx-Ir@rmk-PC.armlinux.org.uk


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