net: mqtt-sn: Correct the allowed Keep Alive value range

Keep Alive timeout is represented by 2-byte unsigned integer, however
the corresponding Kconfig option was limited to UINT8_MAX only.
Also, similarly to regular MQTT, allow to disable the Keep Alive
functionality by specifying the Keep Alive value to 0.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>