kernel test robot rightly points out that w1_poll_completion() should be
static, so mark it as such.

Cc: Ivan Zaentsev <ivan.zaentsev@wirenboard.ru>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20201005123703.GA800532@kroah.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>