media: camss: mark PM functions as __maybe_unused

The empty suspend/resume functions cause a build warning
when they are unused:

drivers/media/platform/qcom/camss/camss.c:1001:12: error: 'camss_runtime_resume' defined but not used [-Werror=unused-function]
drivers/media/platform/qcom/camss/camss.c:996:12: error: 'camss_runtime_suspend' defined but not used [-Werror=unused-function]

Mark them as __maybe_unused so the compiler can silently drop them.

Fixes: 02afa816

 ("media: camss: Add basic runtime PM support")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Todor Tomov <todor.tomov@linaro.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>