hwmon: Fix possible NULL pointer

The recent addition of the label attribute added some code that read the
"label" device property, without checking first that "dev" was non-NULL.

Fix this issue by first checking that "dev" is non-NULL.

Fixes: ccd98cba6a18 ("hwmon: Add "label" attribute")
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>