Increase SPI transaction buffer size from 8MB to 12MB.
Increase SPI transaction buffer size from 8MB to 12MB.

This is needed to support our dish ditector, which is 11.7 MB. In addition, increased timeout from 10s to 15s. I got 15 by 10/8MB*12MB. Let me know if I need to use a different number.

Tested:
Run image_classification, object_detection, and face_detection demo with new values.

Change-Id: Ia0569331384efea664d536ecbd795ea737be8284

google/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian