[PATCH v3] iio: dac: max5821: fix return value check in powerdown sync

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

The function max5821_sync_powerdown_mode() returned the result of
i2c_master_send() directly. If a partial transfer occurred, it would
be incorrectly treated as a success by the caller.

While the caller currently handles the positive return value of 2 as
success, this patch refactors the function to return 0 on full success
and -EIO on short writes. This ensures robust error handling for
incomplete transfers and improves code maintainability by using
sizeof(outbuf).

Fixes: 472988972737 ("iio: add support of the max5821")
Signed-off-by: Salah Triki <salah.triki@gmail.com>
---
Changes in v3:
- Remove blank lines

Changes in v2:
- Replace magic number '2' with 'sizeof(outbuf)' for better maintainability.
- Remove redundant blank line between i2c_master_send() and error check.
- Update commit message to clarify that the 'Fixes' tag is primarily for
  handling partial transfers (short writes), as the caller already
  handled the success case correctly.

 drivers/iio/dac/max5821.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/dac/max5821.c b/drivers/iio/dac/max5821.c
index e7e29359f8fe..dd4e35460195 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 +104,13 @@ 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, sizeof(outbuf));
+	if (ret < 0)
+		return ret;
+	if (ret != sizeof(outbuf))
+		return -EIO;
+
+	return 0;
 }
 
 static ssize_t max5821_write_dac_powerdown(struct iio_dev *indio_dev,
-- 
2.43.0