iio: health: max30102: do not use internal iio_dev lock

The pattern used in this device does not quite fit in the
iio_device_claim_direct_mode() typical usage. In this case, we want to
know if we are in buffered mode or not to know if the device is powered
(buffer mode) or not. And depending on that max30102_get_temp() will
power on the device if needed. Hence, in order to keep the same
functionality, we try to:

1. Claim Buffered mode;
2: If 1) succeeds call max30102_get_temp() without powering on the
device;
3: Release Buffered mode;
4: If 1) fails, Claim Direct mode;
5: If 4) succeeds call max30102_get_temp() with powering on the device;
6: Release Direct mode;
7: If 4) fails, goto to 1) and try again.

This dance between buffered and direct mode is not particularly pretty
(as well as the loop introduced by the goto statement) but it does allow
us to get rid of the mlock usage while keeping the same behavior.

Signed-off-by: Nuno Sá <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/health/max30102.c

···
 		 * Temperature reading can only be acquired when not in
 		 * shutdown; leave shutdown briefly when buffer not running
 		 */
-		mutex_lock(&indio_dev->mlock);
-		if (!iio_buffer_enabled(indio_dev))
+any_mode_retry:
+		if (iio_device_claim_buffer_mode(indio_dev)) {
+			/*
+			 * This one is a *bit* hacky. If we cannot claim buffer
+			 * mode, then try direct mode so that we make sure
+			 * things cannot concurrently change. And we just keep
+			 * trying until we get one of the modes...
+			 */
+			if (iio_device_claim_direct_mode(indio_dev))
+				goto any_mode_retry;
+
 			ret = max30102_get_temp(data, val, true);
-		else
+			iio_device_release_direct_mode(indio_dev);
+		} else {
 			ret = max30102_get_temp(data, val, false);
-		mutex_unlock(&indio_dev->mlock);
+			iio_device_release_buffer_mode(indio_dev);
+		}
 		if (ret)
 			return ret;