iot/zoap: Increase the range of timeouts

To avoid the risk of overflowing when dealing with retransmission
timeouts, increase the size of their representation to 32-bits.

Change-Id: I7c9c1e00c41f5ba75e19b0fd4527f273852eb85f
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
2 files changed