ExternalCameraDevice: increase max bytes per pixel

This fixes some USB external camera which fails to initialize with:

  I ExtCamDevSsn@3.4: configureV4l2StreamLocked: V4L2 buffer size is 614989
  E ExtCamDevSsn@3.4: configureV4l2StreamLocked: V4L2 buffer size: 614989 looks invalid. Expected maximum size: 614400
  E ExtCamDevSsn@3.4: V4L configuration failed!, format:MJPG, w 640, h 480

BUG=b:326773369, b:309244873
Test: Manual camera operation verification

Change-Id: I532bef429792f5a841034a4f3d4eadfa6ed78b2a
Reviewed-by: Tomasz Nowicki <tnowicki@google.com>
Tested-by: Tomasz Nowicki <tnowicki@google.com>
Reviewed-by: Konrad Adamczyk <konrada@google.com>