net: socket: services: Restore only the socket related to work

Restore only the socket descriptor that we marked as -1 after running
the work related to that socket. Earlier we tried to restore the whole
global array of descriptors which could go wrong and is not needed
as we only support synchronous work.

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