lib: pmu: check SSCOF before masking

We should check if SSCOF extension is present,
before applying inhibit mask and clearing overflow,
otherwise undesirable value can be written
in MHPMEVENT_N CSR.

Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Xiang W <wxjstz@126.com>