lib: utils: fdt_pmu: Do not iterate over the fdt_pmu_evt_select table

The valid entry count is tracking by hw_event_count so there
is no need to check the whole table.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>