net: stmmac: visconti: reorganise visconti_eth_set_clk_tx_rate()

Rather than testing dwmac->phy_intf_sel several times for the same
values in this function, group the code together. The only part
which was common was stopping the internal clock before programming
the clock setting.

This further improves the readability of this function.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uRH26-004UyM-9G@rmk-PC.armlinux.org.uk


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