powerpc: Fix doorbell type shift

doorbell type is defined as bits 32:36 so should be shifted by 63-36 =
27 rather than 28.

We never noticed this bug as we've only every used type PPC_DBELL = 0.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>