[media] radio/si470x/radio-si470x.h: use IS_ENABLED() macro

replace:
 #if defined(CONFIG_USB_SI470X) || \
     defined(CONFIG_USB_SI470X_MODULE)
with:
 #if IS_ENABLED(CONFIG_USB_SI470X)
This change was made for: CONFIG_USB_SI470X,
CONFIG_I2C_SI470X

Reported-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Peter Senna Tschudin <peter.senna@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>