blobmsg_json: fix int16 serialization

blobmsg_json: fix int16 serialization

int16 blobmsg type is currently being serialized as uint16_t due to
missing cast during JSON output.

Following blobmsg content:

 bar-min: -32768 (i16)
 bar-max: 32767 (i16)

Produces following JSON:

 { "bar-min":32768,"bar-max":32767 }

Whereas one would expect:

 { "bar-min":-32768,"bar-max":32767 }

Reviewed-by: Jo-Philipp Wich <[email protected]>
Signed-off-by: Petr Štetiar <[email protected]>