fujitsu-laptop: Support radio toggle button

Lifebook E734/E744/E754 has a radio toggle button which uses code 0x420.
Map it to KEY_RFKILL.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Acked-by: Jonathan Woithe <jwoithe@just42.net>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>