V4L/DVB (7671): pxa-camera: fix DMA sg-list coalescing for more than 2 buffers

Currently the pxa-camera driver has a bug, visible when the user requests
more than 2 video buffers. When the third buffer is queued, it is not
appended to the DMA-descriptor list of the second buffer, but is again
appended to the first buffer. Fix it.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>