[media] em28xx: move field 'pos' from struct em28xx_dmaqueue to struct em28xx_buffer

This field is used to keep track of the current memory position in the buffer,
not in the dma queue, so move it to right place.
This also allows us to get rid of the struct em28xx_dmaqueue pointer parameter
in functions em28xx_copy_video() and em28xx_copy_vbi().

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>