net: mqtt: use MQTT_UTF8_LITERAL() to simplify code and fix debug log

Contents of mqtt_3_1_0_proto_desc and mqtt_3_1_1_proto_desc were logged
with following code:

  MQTT_TRC("Encoding Protocol Description. Str:%s Size:%08x.",
           mqtt_proto_desc->utf8, mqtt_proto_desc->size);

This resulted in invalid log, since they were not NULL-terminated
strings. Use MQTT_UTF8_LITERAL() to initialize both utf8 strings to make
sure they are NULL-terminated now and valid to print and
log. Additionally this makes the code a bit shorter.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>