Fix uint16_t data being truncated to uint8_t
Fix uint16_t data being truncated to uint8_t

I encountered an issue when I configured PacketBuffer to be 9016 to
support UDP packets with jumbo MTU size. Generating CASE session was
failing with WEAVE_ERROR_BUFFER_TOO_SMALL becase the PacketBuffer's max
data length was being truncated into uint8_t.

openweave/openweave-core,openweave/openweave-core,openweave/openweave-core,openweave/openweave-core,openweave/openweave-core,openweave/openweave-core