cpupower: revise is_valid flag handling for idle_monitor

The is_valid flag should reflect the validity state of both
the XXX_start and XXX_stop functions. But the use of '=' in
XXX_stop overwrites the validity state set by XXX_start. This
commit changes '=' to '|=' in XXX_stop to preserve and combine
the validity state of XXX_start and XXX_stop.

Signed-off-by: wangfushuai <wangfushuai@baidu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>