w1: w1_therm: make w1_poll_completion static

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: Nkernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20201005123703.GA800532@kroah.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/w1/slaves/w1_therm.c

@@ -1031,7 +1031,7 @@ static int reset_select_slave(struct w1_slave *sl)
  *
  * Return: 0 - OK, negative error - timeout
  */
-int w1_poll_completion(struct w1_master *dev_master, int tout_ms)
+static int w1_poll_completion(struct w1_master *dev_master, int tout_ms)
 {
 	int i;