net: sockets: Do not start service thread if too little resources

If the CONFIG_NET_SOCKETS_POLL_MAX is smaller than what is needed
for the socket service API to work properly, then we should not
start the service thread as the service API cannot work and might
cause memory overwrite in ctx.events[] array.

Fixes #69233

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>