[POWERPC] Invert null match behaviour for irq_hosts

Currently if you don't specify a match callback for your irq_host it's
assumed you match everything. This is a kind of opt-out approach, and
turns out to be the exception rather than the rule.

So change the semantics to be opt-in, ie. you don't match anything unless
you provide a match callback.  This in itself isn't very useful, but will
allow us to provide a default match implementation in a subsequent patch.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>