[POWERPC] Simplify error logic in u3msi_setup_msi_irqs()

u3msi_setup_msi_irqs() doesn't need to call teardown() itself,
the generic code will do this for us as long as we return a non
zero value.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>