hwmon: (sht3x) Fix error handling

Handling of errors when reading status, temperature, and humidity returns
the error number as negative attribute value. Fix it up by returning
the error as return value.

Fixes: a0ac418c ("hwmon: (sht3x) convert some of sysfs interface to hwmon")
Cc: JuenKit Yip <JuenKit_Yip@hotmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>