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: f2a0bd37

 ("[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>