powerpc: Fix memory leak in axon_msi.c

cppcheck found a memory leak in axon_msi, if dcr_base or dcr_len are zero,
we have already allocated msic, so we should free it in the error path.

Signed-off-by: Eric Sesterhenn <eric.sesterhenn@lsexperts.de>
Acked-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>