V4L/DVB (4953): Usbvision minor fixes

- fix debug outputs
- fix returned parameters on VIDIOC_G_FMT, VIDIOC_S_FMT and 
  VIDIOC_TRY_FMT and mmap size setting

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>