iio: adc: ad7192: Grab direct mode for calibration

While a calibration is running, better don't make the device do anything
else.

To enforce that, grab direct mode during calibration.

Fixes: 42776c14c692 ("staging: iio: adc: ad7192: Add system calibration support")
Signed-off-by: Uwe Kleine-König <[e-mail]>
Link: https://patch.msgid.link/8aade80[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/ad7192.c

···
 	if (ret)
 		return ret;
 
+	if (!iio_device_claim_direct(indio_dev))
+		return -EBUSY;
+
 	temp = st->syscalib_mode[chan->channel];
 	if (sys_calib) {
 		if (temp == AD7192_SYSCALIB_ZERO_SCALE)
···
 			ret = ad_sd_calibrate(&st->sd, AD7192_MODE_CAL_SYS_FULL,
 					      chan->address);
 	}
+
+	iio_device_release_direct(indio_dev);
 
 	return ret ? ret : len;
 }