lib: sbi_pmu: Avoid out of bounds access

On a misconfigured system we could access phs->active_events[] out of
bounds. Check that num_hw_ctrs is less or equal SBI_PMU_HW_CTR_MAX.

Addresses-Coverity-ID: 1566113 ("Out-of-bounds read")
Addresses-Coverity-ID: 1566114 ("Out-of-bounds write")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>