perf/x86/intel/uncore: Clean up error handling path of iio mapping

The error handling path of iio mapping looks fragile. We already fixed
one issue caused by it, commit f797f05d

 ("perf/x86/intel/uncore:
Fix for iio mapping on Skylake Server"). Clean up the error handling
path and make the code robust.

Reported-by: gushengxian <gushengxian@yulong.com>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/40e66cf9-398b-20d7-ce4d-433be6e08921@linux.intel.com