ARM: OMAP4+: PRM: fix of_irq_get() result checks

of_irq_get() may return 0 as well as a nagative error number on failure,
(and never on success), however omap44xx_prm_late_init() regards 0 as a
valid IRQ -- fix this.

Fixes: a8f83aef ("ARM: OMAP4+: PRM: register interrupt information from DT")
Fixes: c5b39558 ("ARM: OMAP4: Fix legacy code clean-up regression")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>