[PATCH] iio: dac: max5821: fix return value check in powerdown sync

[Salah Triki <salah.triki@gmail.com>]

The function max5821_sync_powerdown_mode() previously returned the
result of i2c_master_send() directly. This is problematic because:

1. A successful I2C transfer returns the number of bytes sent (2),
   but the caller might expect 0 for success.
2. If a partial transfer occurs (e.g., only 1 byte sent), it would
   be treated as success by the caller checking for negative values.

This patch refactors the return value handling to:
- Propagate negative error codes from the I2C core.
- Return -EIO in case of incomplete transfers (short writes).
- Return 0 on complete success.

Fixes: 472988972737 ("iio: add support of the max5821")
Signed-off-by: Salah Triki <salah.triki@gmail.com>
---
 drivers/iio/dac/max5821.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/dac/max5821.c b/drivers/iio/dac/max5821.c
index e7e29359f8fe..bd14f578d8c5 100644
--- a/drivers/iio/dac/max5821.c
+++ b/drivers/iio/dac/max5821.c
@@ -90,6 +90,7 @@ static int max5821_sync_powerdown_mode(struct max5821_data *data,
 				       const struct iio_chan_spec *chan)
 {
 	u8 outbuf[2];
+	int ret;
 
 	outbuf[0] = MAX5821_EXTENDED_COMMAND_MODE;
 
@@ -103,7 +104,15 @@ static int max5821_sync_powerdown_mode(struct max5821_data *data,
 	else
 		outbuf[1] |= MAX5821_EXTENDED_POWER_UP;
 
-	return i2c_master_send(data->client, outbuf, 2);
+	ret = i2c_master_send(data->client, outbuf, 2);
+
+	if (ret < 0)
+		return ret;
+
+	if (ret != 2)
+		return -EIO;
+
+	return 0;
 }
 
 static ssize_t max5821_write_dac_powerdown(struct iio_dev *indio_dev,
-- 
2.43.0