thunderbolt: Make pathname to force_power shorter

WMI is the bus inside kernel, so, we may access the GUID via
/sys/bus/wmi instead of doing this through /sys/devices path.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mario Limonciello <mario.limonciello@dell.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>