irq: mips: simplify bcm6345_l1_irq_handle()

As bcm6345_l1_irq_handle() only needs to know /whether/ an IRQ was
resolved, and doesn't need to know the specific IRQ, it's simpler for it
to call generic_handle_domain_irq() directly and check the return code,
so let's do that.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Suggested-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Thomas Gleixner <tglx@linutronix.de>