[media] bttv: move fini_bttv_i2c() from bttv-input.c to bttv-i2c.c

Like init_bttv_i2c(), fini_bttv_i2c() belongs to bttv-i2c.c.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>