Both spapr_irq_alloc() and spapr_irq_alloc_block() have an errp
parameter, but they don't use it if XICS hasn't been initialized
yet.

This is doubly wrong:

- all callers do pass a non-null Error **, ie, they expect an error
  to be propagated in case of failure

- XICS obviously needs to be initialized before anything starts allocating
  IRQs

So this patch turns the check into an assert.
Signed-off-by: NGreg Kurz <groug@kaod.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>