HID: cougar: Add support for the Cougar 500k Gaming Keyboard

Cougar 500k Gaming Keyboard have some special function keys that
make the keyboard stop responding once pressed. Implement the custom
vendor interface that deals with the extended keypresses to fix.

The bug can be reproduced by plugging in the keyboard, then pressing the
rightmost part of the spacebar.

Signed-off-by: Daniel M. Lambea <dmlambea@gmail.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>