iio: adc: ad7791: Use sysfs_match_string() helper

Use sysfs_match_string() helper instead of open coded variant.

Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/ad7791.c

@@ -272,11 +272,9 @@ static ssize_t ad7791_write_frequency(struct device *dev,
 	struct ad7791_state *st = iio_priv(indio_dev);
 	int i, ret;
 
-	for (i = 0; i < ARRAY_SIZE(ad7791_sample_freq_avail); i++)
-		if (sysfs_streq(ad7791_sample_freq_avail[i], buf))
-			break;
-	if (i == ARRAY_SIZE(ad7791_sample_freq_avail))
-		return -EINVAL;
+	i = sysfs_match_string(ad7791_sample_freq_avail, buf);
+	if (i < 0)
+		return i;
 
 	ret = iio_device_claim_direct_mode(indio_dev);
 	if (ret)