net: stmmac: starfive: use generic stmmac_set_clk_tx_rate()

Use the generic stmmac_set_clk_tx_rate() to configure the MAC transmit
clock.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1tna0V-0052sk-1L@rmk-PC.armlinux.org.uk


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