[media] tw68: refactor and cleanup the tw68 driver

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>