hwmon: (lm80) fix a missing check of the status of SMBus read

[ Upstream commit c9c63915519b1def7043b184680f33c24cd49d7b ]

If lm80_read_value() fails, it returns a negative number instead of the
correct read data. Therefore, we should avoid using the data if it
fails.

The fix checks if lm80_read_value() fails, and if so, returns with the
error number.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
[groeck: One variable for return values is enough]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>