lib: sbi_pmu: Fix the counter info function

The counter info should only return valid hardware counters for the ones
set in the counter mask. Otherwise, it will report incorrect number of
hardware counters to the supervisor if the platform has discontiguous
counters.

Fixes: c744ed77

 ("lib: sbi_pmu: Enable noncontigous hpm event and counters")
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>