staging: iio: adc: ad7192: use driver private lock to protect hardware state changes

The IIO subsystem is redefining iio_dev->mlock to be used by
the IIO core only for protecting device operating mode changes.
ie. Changes between INDIO_DIRECT_MODE, INDIO_BUFFER_* modes.

In this driver, mlock was being used to protect hardware state
changes. Replace it with a driver private lock.

Also, as there are state changes in the ad7192_ write_raw function, a lock
is added to prevent the concurrent state changes.

Signed-off-by: Aastha Gupta <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/staging/iio/adc/ad7192.c

···
 	u32				scale_avail[8][2];
 	u8				gpocon;
 	u8				devid;
+	struct mutex			lock;	/* protect sensor state */
 
 	struct ad_sigma_delta		sd;
 };
···
 	case IIO_CHAN_INFO_SCALE:
 		switch (chan->type) {
 		case IIO_VOLTAGE:
-			mutex_lock(&indio_dev->mlock);
+			mutex_lock(&st->lock);
 			*val = st->scale_avail[AD7192_CONF_GAIN(st->conf)][0];
 			*val2 = st->scale_avail[AD7192_CONF_GAIN(st->conf)][1];
-			mutex_unlock(&indio_dev->mlock);
+			mutex_unlock(&st->lock);
 			return IIO_VAL_INT_PLUS_NANO;
 		case IIO_TEMP:
 			*val = 0;
···
 	switch (mask) {
 	case IIO_CHAN_INFO_SCALE:
 		ret = -EINVAL;
+		mutex_lock(&st->lock);
 		for (i = 0; i < ARRAY_SIZE(st->scale_avail); i++)
 			if (val2 == st->scale_avail[i][1]) {
 				ret = 0;
···
 				ad7192_calibrate_all(st);
 				break;
 			}
+		mutex_unlock(&st->lock);
 		break;
 	case IIO_CHAN_INFO_SAMP_FREQ:
 		if (!val) {
···
 		return -ENOMEM;
 
 	st = iio_priv(indio_dev);
+
+	mutex_init(&st->lock);
 
 	st->avdd = devm_regulator_get(&spi->dev, "avdd");
 	if (IS_ERR(st->avdd))