Refactor and clean up the tw68 driver. It's now using the proper
V4L2 core frameworks.

Tested with my Techwell tw6805a and tw6816 grabber boards.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>