logging: backend: net: avoid early enabling

Avoid early enabling of the syslog backend in
the the dhcpv4 options parser.

When CONFIG_NET_IPV4_ACD is enabled, the assigned ip address has
not been checked, when the other dhcpv4 options are parsed, this would
lead to the syslog backend being enabled before the src ip address
is valid, so we get lots of warnings about dropd packets, this fixes
it at least on start. We will still get the warnings, when the iface
goes down and then up later.

Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>