bug fix: can_datahandler wrong return value

can_datahandler returns an unsigned value that represent the
number of bytes actually bufffered, but the call to iob_tryadd_queue
returns a negative value when it fails so that value can't be used as
can_datahandler return value because it is interpreted as unsigned so
a big number is wrongly returned when iob_tryadd_queue fails.

The fix is just setting the return value to zero when iob_tryadd_queue
fails, reflecting the actual meaning that no bytes have been buffered.

net/can/can_callback.c

@@ -234,6 +234,7 @@ uint16_t can_datahandler(FAR struct net_driver_s *dev,
   else
     {
       nerr("ERROR: Failed to queue the I/O buffer chain: %d\n", ret);
+      ret = 0;
       goto errout;
     }