net: lwm2m: Fix socket hints for block transfer

Outgoing block-transfers now set the socket hint
to ONGOING as long as the BLOCK1/BLOCK2 header has
MORE flag set to true.
This means as only the last packet in the block-transfer
set the socket hint to LAST or ONE_RESPONSE.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>