net: lwm2m: optimize `socket_loop` by using local variables

Changes include:
- Introducing a local `ctx` variable to replace multiple
  `sock_ctx[i]` references.
- Using a local `revents` variable to simplify repeated
  `sock_fds[i].revents` checks.
- Consolidating conditional checks for socket events
  (ZSOCK_POLLERR, ZSOCK_POLLNVAL, and ZSOCK_POLLHUP) as they are
  individual bit definitions, allowing them to be checked simultaneously.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>