V4L/DVB (6914): Fix a few issues at the bttv conversion

bttv driver is working as previously. An additional gain of about 1,5Kb were
obtained with the driver conversion to vidioc_ioctl2:

   text    data     bss     dec     hex filename
  89208   40244   57556  187008   2da80 old/bttv.ko
  88960   38740   57556  185256   2d3a8 new/bttv.ko

(measured on a x86_64)

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>