bts: In case the line isn't created do not exit with code 1
bts: In case the line isn't created do not exit with code 1

The service file will prevent a re-launch of the sysmobts.service
in case the main process exits with '1'. In case the ethernet is
not available yet the routine would fail and the sysmobts process
will not be restarted.

unable to connect/bind socket: Network is unreachable
<000f> input/ipaccess.c:885 cannot open OML BTS link: Network is unreachable
unable to connect to BSC
root@sysmobts-v2:~# echo $?
2

Fixes: SYS#736

osmocom/osmo-bts,geosphere/osmo-bts,geosphere/osmo-bts,geosphere/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts,geosphere/osmo-bts