staging: vt6655: Declare vnt_init() static

Fix sparse warning:
  drivers/staging/vt6655/device_main.c:1672:5: warning: symbol 'vnt_init' was not declared. Should it be static?

Signed-off-by: Sören Brinkmann <soeren.brinkmann@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>