OMAP2/3: omap mailbox: platform_get_irq() error ignored

platform_get_irq may return -ENXIO. but struct omap_mbox mbox_dsp_info.irq
is unsigned, so the error was not noticed.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>