net: stmmac: rk: simplify set_*_speed()

net: stmmac: rk: simplify set_*_speed()

Rather than having lots of regmap_write()s to the same register but
with different values depending on the speed, reorganise the
functions to use a local variable for the value, and then have one
regmap_write() call to write it to the register. This reduces the
amount of code and is a step towards further reducing the code size.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uPk2o-004CFH-Q4@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>