hwmon: (adt7475) Make adt7475_read_word() return errors

The adt7475_read_word() function was meant to return negative error
codes on failure.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Tokunori Ikegami <ikegami@allied-telesis.co.jp>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>