ACPI: processor idle: Use swap() instead of open coding it

Address the following coccicheck warning:

./drivers/net/ethernet/mellanox/mlx5/core/en/tc_ct.c:914:40-41:
WARNING opportunity for swap().

by using swap() for the swapping of variable values and drop
the tmp variable that is not needed any more.

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
[ rjw: Subject and changelog rewrite ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>