powerpc/via-pmu: fix error path in find_via_pmu()

Cleanup was not in the reverse order from the set-up, so not all
the gotos made sense, and also it was being avoided completely upon
failure of init_pmu().

Signed-off-by: Phil Carmody <pc+lkml@asdf.org>
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>