[media] s5p_cec: mark suspend/resume as __maybe_unused

The suspend/resume functions in the s5p-cec driver are only
referenced when CONFIG_PM is enabled, so we get a warning
about unused functions otherwise:

drivers/staging/media/s5p-cec/s5p_cec.c:260:12: error: 's5p_cec_resume' defined but not used [-Werror=unused-function]
 static int s5p_cec_resume(struct device *dev)
            ^~~~~~~~~~~~~~
drivers/staging/media/s5p-cec/s5p_cec.c:253:12: error: 's5p_cec_suspend' defined but not used [-Werror=unused-function]
 static int s5p_cec_suspend(struct device *dev)

This marks them as __maybe_unused to avoid the warning without
having to introduce an extra #ifdef.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>