net: stmmac: visconti: re-arrange speed decode

Re-arrange the speed decode in visconti_eth_set_clk_tx_rate() to be
more readable by first checking to see if we're using RGMII or RMII
and then decoding the speed, rather than decoding the speed and then
testing the interface mode.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uRH21-004UyG-50@rmk-PC.armlinux.org.uk


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