V4L/DVB (7565): em28xx: fix buffer underrun handling

This patch fixes three related issues and a fourth trivial one:

- Use buffers even if no-one's currently waiting for them (fixes
  underrun issues);

- Don't return incomplete/mangled frames at the start of streaming and
  in the case of buffer underruns;

- Fix an issue which could cause the driver to write to a buffer that's
  been freed after videobuf_queue_cancel is called (exposed by the
  previous two fixes - for some reason, ignoring buffers that weren't
  being waited on worked around the issue);

- Fix a bug which could cause only one field to be filled in the first
  buffer (or first few buffers) after streaming is started.

Signed-off-by: Aidan Thornton <makosoft@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>