configurenetwork: Put One- and Two-Port pre-setup into function

This is a cosmetical patch, however it does increase the overview
a lot in my opinion.

We cannot drop the special case of the two-port devices, because
we need the current setup to enable setting port mode in the Web
UI.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>