media: bttv: refactor bttv_set_dma()

Break bttv_set_dma() into several smaller, separate functions so it is
easier to read the risc and dma code. Replace numeric values with
descriptive macros. Also remove the unused field btv->cap_ctl.

Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>