ARM: OMAP3+: PRM: fix of_irq_get() result check

of_irq_get() may return 0 as well as a nagative error number on failure
(and never on success), however omap3xxx_prm_late_init() regards 0 as a
valid IRQ -- fix this.

Fixes: 1e037794 ("ARM: OMAP3+: PRM: register interrupt information from DT")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>