lib: sbi_pmu: avoid buffer overflow

total_ctrs is bounded by

    SBI_PMU_FW_CTR_MAX + SBI_PMU_HW_CTR_MAX) == 48

which exceeds BITS_PER_LONG on 32 bit systems.

Iterating over the bits of &cmask results in a buffer overflow when looking
for a bit >= BITS_PER_LONG.

Adjust the iterators in sbi_pmu_ctr_start() and sbi_pmu_ctr_stop()
accordingly.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Anup Patel <anup@brainfault.org>