samples: bmi160: reduce polling mode sampling frequency

Reduce the accelerometer sampling frequency for polling mode.
So, cpu has more time to fetch the sample.

Also add some waiting time for the new frequency setting to
take effect.

Jira: ZEP-1532

Change-Id: If5c1d8a2b5f30b232a212f2775306ca397b9f80c
Signed-off-by: Baohong Liu <baohong.liu@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>