[media] atmel-isc: Set the default DMA memory burst size

Sometimes 'DMA single access' is not enough to transfer
a frame of image, '8-beat burst access' is set as the
default DMA memory burst size.

Signed-off-by: Songjun Wu <songjun.wu@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>