perf hwmon_pmu: Ensure hwmon key union is zeroed before use

Non-zero values led to mismatches in testing. This was reproducible
with -fsanitize=undefined.

Reported-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Closes: https://lore.kernel.org/lkml/Zzdtj0PEWEX3ATwL@x1/


Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20241119230033.115369-1-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>