sensor-cache: Read sensor data when there is no cache

When IPMI starts and the sensor has no match callback, the sensor cache
is empty. Read the sensor data in such case and cache it.

Tested: Verify the sensor list works as expected, the first sensor
        list takes longer time, and the following sensor list takes less
        time because of the cache.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: Icc33185b93cf21462584b3817b7c522e24b70baf
2 files changed