powerpc/8xx: fix mpc8xx_get_irq() return on no irq

IRQ 0 is a valid HW interrupt. So get_irq() shall return 0 when
there is no irq, instead of returning irq_linear_revmap(... ,0)

Fixes: f2a0bd3 ("[POWERPC] 8xx: powerpc port of core CPM PIC")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/sysdev/mpc8xx_pic.c

@@ -79,7 +79,7 @@ unsigned int mpc8xx_get_irq(void)
 	irq = in_be32(&siu_reg->sc_sivec) >> 26;
 
 	if (irq == PIC_VEC_SPURRIOUS)
-		irq = 0;
+		return 0;
 
         return irq_linear_revmap(mpc8xx_pic_host, irq);