perf/dwc_pcie: Fix use of uninitialized variable

perf/dwc_pcie: Fix use of uninitialized variable

Fix use of uninitialized variable in group validation code.

Fixes: 71396cfac97d ("perf/dwc_pcie: Support counting multiple lane events in parallel")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/r/202509231223.gZsX6Eio-lkp@intel.com/
Signed-off-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Signed-off-by: Will Deacon <will@kernel.org>