uorb: allow for more than 255 uORB messages

Increased size for ORB_ID from uint8_t to uint16_t

Created a type: orb_id_size_t = uint16_t.
There are still a couple of places where the size
of the ORB_ID is assumed to be less than 16-bits.
The places that I have found are commented regarding
this and can be found with a search on orb_id_size_t.

platforms/common/uORB/uORBDeviceMaster.hpp

@@ -98,7 +98,7 @@ public:
 			return false;
 		}
 
-		return _node_exists[instance][(uint8_t)id];
+		return _node_exists[instance][(orb_id_size_t)id];
 	}
 
 	/**