samples: net: cellular_modem: small fixes/improvements

- A small delay is added after the post-restart L4_CONNECTED event
  is received to prevent trying to send packets too quickly. Otherwise
  the first packet would not get sent and the sample would hang on recv().
- Some logs and function parameters are fixed.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>