hp-wmi: Enable hotkeys on some systems

Kyle Evans discovered that he needed to set some bits in an EC register in
order to receive hotkey events. Doing so blindly broke some otherwise
working HP laptops. It turns out that there's a WMI call that accesses
the same register, so let's try calling that instead.

Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>
Cc: Kyle Evans <kvans32@gmail.com>