net: stmmac: ethqos: clean up setting serdes speed

There are four repititions of the same sequence of code, three of which
are identical. Pull these out into a separate function to improve
readability.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Halaney <ahalaney@redhat.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/E1sCErj-00EOQ9-Vh@rmk-PC.armlinux.org.uk


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