w1: w1_io.c: fix a kernel-doc warning

Add a blank line to avoid this Sphinx warning:
	./drivers/w1/w1_io.c:197: WARNING: Definition list ends without a blank line; unexpected unindent.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>