irqchip: omap-intc: correct maximum number or MIR registers

maximum number of MIR register is 4, rather than 3.
Fix that.

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