[media] use CONFIG_PM_SLEEP for suspend/resume

Using CONFIG_PM_SLEEP suppress the warnings when the driver is
compiled without PM sleep functions:

drivers/media/rc/st_rc.c:338:12: warning: ‘st_rc_suspend’ defined but not used [-Wunused-function]
drivers/media/rc/st_rc.c:359:12: warning: ‘st_rc_resume’ defined but not used [-Wunused-function]

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>