iio: event_monitor: report unsupported events

This makes the event monitor bail out with a helpful error
message if a device does not support events, as a related
fix to iio core now makes it return -ENODEV properly.

Signed-off-by: Linus Walleij <[e-mail]>
Acked-by: Hartmut Knaack <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

tools/iio/iio_event_monitor.c

···
 	ret = ioctl(fd, IIO_GET_EVENT_FD_IOCTL, &event_fd);
 	if (ret == -1 || event_fd == -1) {
 		ret = -errno;
-		fprintf(stderr, "Failed to retrieve event fd\n");
+		if (ret == -ENODEV)
+			fprintf(stderr,
+				"This device does not support events\n");
+		else
+			fprintf(stderr, "Failed to retrieve event fd\n");
 		if (close(fd) == -1)
 			perror("Failed to close character device file");