[POWERPC] Simplify error logic in rtas_setup_msi_irqs()

rtas_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>