sh: intc: Fix up build failure introduced by radix tree changes.

The radix tree retry logic got a bit of an overhaul and subsequently
broke the virtual IRQ subgroup build. Simply switch over to
radix_tree_deref_retry() as per the filemap changes, which the virq
lookup logic was modelled after in the first place.

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