iio: percolate error if event fd fails

This makes the error from iio_event_getfd() percolate up
to userspace properly so we can know for sure there is no
events on this device (-ENODEV returned). Before this patch
we would bail out looking for the unsupported events on the
erroneous (negative) file descriptor.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-core.c

@@ -1153,6 +1153,8 @@ static long iio_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 
 	if (cmd == IIO_GET_EVENT_FD_IOCTL) {
 		fd = iio_event_getfd(indio_dev);
+		if (fd < 0)
+			return fd;
 		if (copy_to_user(ip, &fd, sizeof(fd)))
 			return -EFAULT;
 		return 0;