pmu->irq_affinity will not be freed if an error occurred within
arm_pmu_device_probe after of_pmu_irq_cfg has been called.

Note that in the case of_pmu_irq_cfg is returning an error,
pmu->irq_affinity will not be set, but it should be NULL as pmu was
kzalloc'd. Therefore the result kfree(NULL) is benign.
Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>