Input: wistron - add LED support

Add support to wistron_btns for leds that come with the multimedia keys.
Mail and wifi leds are supported, on laptops which have them.

Depending on the laptop, wifi subsystem may control just the led, or both
the led and the wifi card. Wifi led interface is activated only for the
former type of laptops, as the latter type is already managed. Leds are
controled by the interface in /sys/class/leds.

Signed-off-by: Eric Piel <eric.piel@tremplin-utc.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>