net: ocpp: Fix multiple issues detected by clang

Fix multiple warnings detected by clang:
* implicit conversions back and forth from enumeration type
  'enum ocpp_notify_reason' to 'enum ocpp_pdu_msg'
  in ocpp_internal_handler()
* label followed by a declarations
* variable declared after one switch case and used in another
* mapping a int to uint and later using it in < 0 comparisons
  and with negative values

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>