perf hwmon_pmu: Fix uninitialized variable warning

perf hwmon_pmu: Fix uninitialized variable warning

The line_len is only set on success. Check the return value instead.

 util/hwmon_pmu.c: In function ‘perf_pmus__read_hwmon_pmus’:
 util/hwmon_pmu.c:742:20: warning: ‘line_len’ may be used uninitialized [-Wmaybe-uninitialized]
   742 |                 if (line_len > 0 && line[line_len - 1] == '\n')
       |                    ^
 util/hwmon_pmu.c:719:24: note: ‘line_len’ was declared here
   719 |                 size_t line_len;

Fixes: 53cc0b351ec9 ("perf hwmon_pmu: Add a tool PMU exposing events from hwmon in sysfs")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>