irq: mips: simplify do_domain_IRQ()

There's no need fpr arch/mips's do_domain_IRQ() to open-code the NULL
check performed by handle_irq_desc(), nor the resolution of the desc
performed by generic_handle_domain_irq().

Use generic_handle_domain_irq() directly, as this is functioanlly
equivalent and clearer.

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

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