net: stmmac: thead: use rgmii_clock() for RGMII clock rate

Switch to using rgmii_clock() to get the RGMII TXC rate, and calculate
the divisor from the parent clock rate and the rate indicated by
rgmii_clock().

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Drew Fustini <drew@pdp7.com>
Link: https://patch.msgid.link/E1tlTo8-004W3a-CO@rmk-PC.armlinux.org.uk


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