V4L/DVB (7488): videobuf: Simplify videobuf_waiton logic and possibly avoid missed wakeup

Possible missed wakeup- use kernel helpers for wait queues
  http://www.mail-archive.com/linux-usb-devel@lists.sourceforge.net/msg27983.html



Signed-off-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>