Move the configuration of 1000Base-X mode into its own function. This
way, redundant code can be replaced by a single function call.
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21184
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>