lib: updatehub: Fix buffer sizes

The MAX_PAYLOAD_SIZE must reflect the size of COAP_BLOCK_x. This is
necessary becase BLOCK size represents max payload size. The current
value create inconsistencies for coap lib. The same way,
MAX_DOWNLOAD_DATA must allocate sufficient space for MAX_PAYLOAD_SIZE
plus all space for coap header etc.

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