[media] dvb_usb_v2: use IS_ENABLED() macro

replace:
 #if defined(CONFIG_RC_CORE) || defined(CONFIG_RC_CORE_MODULE)
with:
 #if IS_ENABLED(CONFIG_RC_CORE)

Reported-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>