[media] media: st-rc: move to using reset_control_get_optional

This patch fixes a compilation error while building with the
random kernel configuration.

drivers/media/rc/st_rc.c: In function 'st_rc_probe':
drivers/media/rc/st_rc.c:281:2: error: implicit declaration of
function 'reset_control_get' [-Werror=implicit-function-declaration]
  rc_dev->rstc = reset_control_get(dev, NULL);

drivers/media/rc/st_rc.c:281:15: warning: assignment makes pointer
from integer without a cast [enabled by default]
  rc_dev->rstc = reset_control_get(dev, NULL);

Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>