perf/x86/intel/uncore: Clear attr_update properly

Current clear_attr_update procedure in pmu_set_mapping() sets attr_update
field in NULL that is not correct because intel_uncore_type pmu types can
contain several groups in attr_update field. For example, SPR platform
already has uncore_alias_group to update and then UPI topology group will
be added in next patches.

Fix current behavior and clear attr_update group related to mapping only.

Fixes: bb42b3d3 ("perf/x86/intel/uncore: Expose an Uncore unit to IIO PMON mapping")
Signed-off-by: Alexander Antonov <alexander.antonov@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20221117122833.3103580-4-alexander.antonov@linux.intel.com