Topstar U931 laptops provide a LED synced with the WLAN adapter
hard-blocking state. Unfortunately, some models seem to be defective,
making impossible to hard-block the adapter with the WLAN switch and
thus the LED is useless.

An ACPI method is available to programmatically control this switch and
it indirectly allows to control the LED.

This commit registers the LED within the corresponding subsystem, making
possible for instance to use an rfkill-based trigger to synchronize the
LED with the device state.

This workaround is enabled automatically for Topstar U931/RVP7 laptops
based on a DMI check.
Signed-off-by: Guillaume Douézan-Grard <gdouezangrard@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>