msi-wmi: Introduced quirk_last_pressed

Introduced quirk_last_pressed variable that would indicate if
last_pressed is used or not. Also converted last_pressed to simple
variable in order to allow keymap to be non-contiguous.

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>