samples: chre: use CONFIG_LOG_MODE_MINIMAL

The sample depends on the log output to check for a successful run. This
means that with a small enough log buffer we could lose messages and
fail the run, as it's happening right now on qemu_cortex_m0.

Switching to CONFIG_LOG_MODE_MINIMAL to avoid the problem in the first
place.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>