firmware: vpd: Tie firmware kobject to device lifetime

It doesn't make sense to have /sys/firmware/vpd if the device is not
instantiated, so tie its lifetime to the device.

Fixes: 049a59db

 ("firmware: Google VPD sysfs driver")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>