staging: use videobuf2 framework for drivers/staging/dt3155v4l driver

This patch transforms drivers/staging/dt3155v4l driver to use videobuf2 framework.
Tested and works with "xawtv -f".

Either streaming API or read method should be selected during kernel configuration.

If both are selected into the driver (not possible without another patching),
either due to my misunderstanding or problems in xawtv (or both), I get kernel panic
after some start/stop of xawtv (not strictly reproducible).

Signed-off-by: Marin Mitov <mitov@issp.bas.bg>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>