irqchip: omap-intc: minor improvement to omap_irq_pending()

We already hold the number of Pending registers
in omap_nr_pending. Let's use that instead.

Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>