MIPS: SGI-IP27: use WARN_ON() output

WARN_ON() propagates the result of conditional expression, and it can be
used to return early in the following expression in the arch_init_irq().

This is a no-op cleanup, except that compiler may optimize the error paths
better because WARN_ON() implies 'unlikely()'.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>