perf: qcom_l2_pmu: ACPI: Use ACPI_COMPANION() directly

The ACPI_HANDLE() macro is a wrapper arond the ACPI_COMPANION()
macro and the ACPI handle produced by the former comes from the
ACPI device object produced by the latter, so it is way more
straightforward to evaluate the latter directly instead of passing
the handle produced by the former to acpi_bus_get_device().

Modify l2_cache_pmu_probe_cluster() accordingly (no intentional
functional impact).

While at it, rename the ACPI device pointer to adev for more
clarity.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>