net: stmmac: rk: simplify px30_set_rmii_speed()

px30_set_rmii_speed() doesn't need to be as verbose as it is - it
merely needs the values for the register and clock rate which depend
on the speed, and then call the appropriate functions. Rewrite the
function to make it so.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uPk3E-004CFl-BZ@rmk-PC.armlinux.org.uk


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