PM / devfreq: Get rid of some doc warnings

Mark "void *data" as literal, in order to avoid those doc warnings:

	./include/linux/devfreq.h:156: WARNING: Inline emphasis start-string without end-string.
	./include/linux/devfreq.h:259: WARNING: Inline emphasis start-string without end-string.
	./include/linux/devfreq.h:279: WARNING: Inline emphasis start-string without end-string.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>