V4L/DVB (7856): cx18/: possible cleanups

This patch contains the following possible cleanups:
- cx18-i2c.c should #include "cx18-i2c.h" for getting the prototypes of
  it's global functions
- make the following needlessly global functions static:
  - cx18-fileops.c:cx18_claim_stream()
  - cx18-fileops.c:cx18_release_stream()
  - cx18-queue.c:cx18_queue_move()
- remove the following unused functions:
  - cx18-driver.c:cx18_waitq()
  - cx18-queue.c:cx18_buf_copy_from_user()

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>