V4L/DVB: pxa-camera: Unsigned dma_chans[] cannot be negative

Unsigned dma_chans[] cannot be negative

Also the third time dma_chans[0] < 0 was tested instead of dma_chans[2]

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>