perf python: Fix thread check in pyrf_evsel__read
The CPU index is incorrectly checked rather than the thread index.
Fixes: 739621f65702 ("perf python: Add evsel read method")
Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250710235126.1086011-11-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
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