net: sockets: socket_service: Fixed issue that prevented reconfiguration

Once running, a socket service could not be reconfigured (i.e. changing
file descriptors and/or events to be polled). This was due to an wrong
check at the end of the thread main loop of socket_service_thread that
evaluated to false as variable ret is zero if trigger_work(), which is
called previously for all returned events, returns successfully.

Signed-off-by: Philipp Finke <philipp_finke@gmx.de>