sh: intc: Handle early lookups of subgroup IRQs.

If lookups happen while the radix node still points to a subgroup
mapping, an IRQ hasn't yet been made available for the specified id, so
error out accordingly. Once the slot is replaced with an IRQ mapping and
the tag is discarded, lookup can commence as normal.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>