[media] dvb-usb: avoid link error with dib3000m{b,c|

Tha ARM randconfig builds came up with another rare build failure
for the dib3000mc driver, when dvb-usb-dibusb-mb is built-in and
dib3000mc is a loadable module:

ERROR: "dibusb_dib3000mc_frontend_attach" [drivers/media/usb/dvb-usb/dvb-usb-nova-t-usb2.ko] undefined!
ERROR: "dibusb_dib3000mc_tuner_attach" [drivers/media/usb/dvb-usb/dvb-usb-nova-t-usb2.ko] undefined!

Apparently this used to be a valid configuration (build-time, not
run-time), but broke as part of a cleanup.

I tried reverting the cleanup, but saw that the code was still wrong
then. This version adds a dependency for dib3000mb, to ensure that
dib3000mb does not force the dibusb_dib3000mc_frontend_attach function
to be built-in when dib3000mc is a loadable module.

I have also checked the two other files that were changed in the original
cleanup, and found them to be correct in either version, so I do not
touch that part.

As this is a rather obscure bug, there is no need for backports.

Fixes: 028c70ff ("[media] dvb-usb/dvb-usb-v2: use IS_ENABLED")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>