mgmt: updatehub: Use coap_packet_get_payload function

The #36758 fixes coap_packet_get_payload function by normalizing
offset value in the library.  That change create a regression on
UpdateHub.  However, the UpdateHub can now use coap_packet_get_payload
function instead manually compute payload start and payload size.
This refactors the current code in favor of coap_packet_get_payload
and fixes the regression.

Signed-off-by: Gerson Fernando Budke <gerson.budke@ossystems.com.br>