[media] ivtv: Fix build warning

In driver ivtv, there is a mismatch between the type of the radio module
parameter and the storage variable, which leads to the following
warning:

  CC [M]  drivers/media/video/ivtv/ivtv-driver.o
drivers/media/video/ivtv/ivtv-driver.c: In function ‘__check_radio’:
drivers/media/video/ivtv/ivtv-driver.c:142: warning: return from incompatible pointer type
drivers/media/video/ivtv/ivtv-driver.c: At top level:
drivers/media/video/ivtv/ivtv-driver.c:142: warning: initialization from incompatible pointer type

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>