iio:ad5064: Make sure ad5064_i2c_write() returns 0 on success

i2c_master_send() returns the number of bytes transferred on success while
the ad5064 driver expects that the write() callback returns 0 on success.
Fix that by translating any non negative return value of i2c_master_send()
to 0.

Fixes: commit 6a17a0768f77 ("iio:dac:ad5064: Add support for the ad5629r and ad5669r")
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/dac/ad5064.c

@@ -598,10 +598,16 @@ static int ad5064_i2c_write(struct ad5064_state *st, unsigned int cmd,
 	unsigned int addr, unsigned int val)
 {
 	struct i2c_client *i2c = to_i2c_client(st->dev);
+	int ret;
 
 	st->data.i2c[0] = (cmd << 4) | addr;
 	put_unaligned_be16(val, &st->data.i2c[1]);
-	return i2c_master_send(i2c, st->data.i2c, 3);
+
+	ret = i2c_master_send(i2c, st->data.i2c, 3);
+	if (ret < 0)
+		return ret;
+
+	return 0;
 }
 
 static int ad5064_i2c_probe(struct i2c_client *i2c,