net: stmmac: thead: switch to use set_clk_tx_rate() hook

Switch from using the fix_mac_speed() hook to set_clk_tx_rate() to
manage the transmit clock.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1tna14-0052tT-S4@rmk-PC.armlinux.org.uk


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