drivers: net: ppp: various fixes

- Disable UART when the PPP interface is brought down.
  This prevents an error when it is next brought up.
- Change the level of certain logs to be less concerning
  and less verbose.
- Fix warnings regarding the passed parameter types of %p conversions.

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