staging:iio:documentation make read_size signed to allow for errors.

Without this all hell breaks loose if you have no data.
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/Documentation/generic_buffer.c

@@ -132,7 +132,7 @@ int main(int argc, char **argv)
 
 	int datardytrigger = 1;
 	char *data;
-	size_t read_size;
+	ssize_t read_size;
 	struct iio_event_data dat;
 	int dev_num, trig_num;
 	char *buffer_access, *buffer_event;