rtc: abx80x: Fix return value of nvmem callback on read

BugLink: https://bugs.launchpad.net/bugs/2083196

commit fc82336b50e7652530bc32caec80be0f8792513b upstream.

Read callbacks registered with nvmem core expect 0 to be returned on
success and a negative value to be returned on failure.

abx80x_nvmem_xfer() on read calls i2c_smbus_read_i2c_block_data() which
returns the number of bytes read on success as per its api description,
this return value is handled as an error and returned to nvmem even on
success.

Fix to handle all possible values that would be returned by
i2c_smbus_read_i2c_block_data().

Fixes: e90ff8ede7 ("rtc: abx80x: Add nvmem support")
Cc: stable@vger.kernel.org
Signed-off-by: Joy Chakraborty <joychakr@google.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>
Link: https://lore.kernel.org/r/20240613120750.1455209-1-joychakr@google.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Portia Stephens <portia.stephens@canonical.com>
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>

drivers/rtc/rtc-abx80x.c

@@ -705,14 +705,18 @@ static int abx80x_nvmem_xfer(struct abx80x_priv *priv, unsigned int offset,
 		if (ret)
 			return ret;
 
-		if (write)
+		if (write) {
 			ret = i2c_smbus_write_i2c_block_data(priv->client, reg,
 							     len, val);
-		else
+			if (ret)
+				return ret;
+		} else {
 			ret = i2c_smbus_read_i2c_block_data(priv->client, reg,
 							    len, val);
-		if (ret)
-			return ret;
+			if (ret <= 0)
+				return ret ? ret : -EIO;
+			len = ret;
+		}
 
 		offset += len;
 		val += len;