[POWERPC] mpic: Make sparse happy

I was running sparse on something else and noticed sparse warnings
and especially the bogus code that is fixed by the first hunk of
this patch, so I fixed them all while at it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Milton Miller <miltonm@bga.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>