ARM: OMAP2+: fix naming collision of variable nr_irqs

Using nr_irqs as local variable name triggers the sparse warning..
./arch/arm/mach-omap2/irq.c:265:6: warning: symbol 'nr_irqs' shadows an earlier one
./linux/include/linux/irqnr.h:26:12: originally declared here

Signed-off-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>