[PATCH] powerpc: Fix setting MPIC priority

Trying to set the priority would just disable the interrupt due to an
incorrect mask used. We rarely use that call, in fact, I think only in
the powermac code for the cmd-power key combo that triggers xmon. So it
got unnoticed for a while.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>