net: coap: add TOO_MANY_REQUESTS to return codes

Add COAP_RESPONSE_CODE_TOO_MANY_REQUESTS to coap_header_get_code(). CoAP
4.29 response code for "Too many requests" is defined in coap.h but was
missing from the list of cases, resulting in a 0 being returned instead
of the proper code.

Signed-off-by: Mike Szczys <mike@golioth.io>