cpuidle: Avoid potential overflow in integer multiplication

cpuidle: Avoid potential overflow in integer multiplication

In detail:

In C language, when you perform a multiplication operation, if
both operands are of int type, the multiplication operation is
performed on the int type, and then the result is converted to
the target type. This means that if the product of int type
multiplication exceeds the range that int type can represent,
an overflow will occur even if you store the result in a
variable of int64_t type.

For a multiplication of two int values, it is better to use
mul_u32_u32() rather than s->exit_latency_ns = s->exit_latency *
NSEC_PER_USEC to avoid potential overflow happenning.

Signed-off-by: C Cheng <C.Cheng@mediatek.com>
Signed-off-by: Bo Ye <bo.ye@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
[ rjw: New subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>