net: coap: Fix coap_packet_get_payload function

The function did not work correct for packets generated with Zephyr
APIs, as `max_len` holds the entire buffer size, not the actual packet
size.

Additionally, unify how Payload Marker is handled in the calculation -
currently the coap parsing function adds it to the `opt_len` field,
which is counter-intuitive.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>