lib: sbi: Fix counter index calculation for SBI_PMU_CFG_FLAG_SKIP_MATCH

As per the SBI specification, we should "unconditionally select the first
counter from the set of counters specified by the counter_idx_base and
counter_idx_mask", so implement this behaviour.

Suggested-by: Atish Patra <atishp@atishpatra.org>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>