zero the stack buffer before giving random garbage to the SCU

some messages take 4 bytes, but only fill 3 bytes....
this patch makes sure that whatever we send to the SCU is zeroed first

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>