net: stmmac: rk: combine clk_mac_speed rate setting functions

rk3568_set_gmac_speed() and rv1126_set_clk_mac_speed() are now
identical. Combine these so we have a single copy of this code.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uPk34-004CFZ-3y@rmk-PC.armlinux.org.uk


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