V4L/DVB: v4l videobuf: use struct videobuf_buffer * instead of void * for videobuf_alloc

videobuf_alloc() returned a void *. Change to struct videobuf_buffer *
to get better type checking.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>