powerpc/xics: Cleanup xics_host_map and ipi

Since we already have a special case in map to set the ipi handler, use
the desired flow.

If we don't find an ics to handle the interrupt complain instead of
returning 0 without having set a chip or handler.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>