[media] media: atmel-isi: increase the burst length to improve the performance

The burst length could be BEATS_4/8/16. Before this patch, isi use default
value BEATS_4. To imporve the performance we could set it to BEATS_16.

Otherwise sometime it would cause the ISI overflow error.

Reported-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>