staging: iio: ad7152: Fix deadlock in ad7152_write_raw_samp_freq()

ad7152_write_raw_samp_freq() is called by ad7152_write_raw() with
chip->state_lock held. So, there is unavoidable deadlock when
ad7152_write_raw_samp_freq() locks the mutex itself.

The patch removes unneeded locking.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Fixes: 6572389b ("staging: iio: cdc: ad7152: Implement
IIO_CHAN_INFO_SAMP_FREQ attribute")
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/cdc/ad7152.c

@@ -231,16 +231,12 @@ static int ad7152_write_raw_samp_freq(struct device *dev, int val)
 	if (i >= ARRAY_SIZE(ad7152_filter_rate_table))
 		i = ARRAY_SIZE(ad7152_filter_rate_table) - 1;
 
-	mutex_lock(&chip->state_lock);
 	ret = i2c_smbus_write_byte_data(chip->client,
 					AD7152_REG_CFG2, AD7152_CFG2_OSR(i));
-	if (ret < 0) {
-		mutex_unlock(&chip->state_lock);
+	if (ret < 0)
 		return ret;
-	}
 
 	chip->filter_rate_setup = i;
-	mutex_unlock(&chip->state_lock);
 
 	return ret;
 }