arm64: perf: fix unassigned cpu_pmu->plat_device when probing PMU PPIs

Commit d795ef9a ("arm64: perf: don't warn about missing
interrupt-affinity property for PPIs") added a check for PPIs so that
we avoid parsing the interrupt-affinity property for these naturally
affine interrupts.

Unfortunately, this check can trigger an early (successful) return and
we will not assign the value of cpu_pmu->plat_device. This patch fixes
the issue.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>