[media] dvb-usb: split out common parts of dibusb

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 tries to fix the code properly, by moving the problematic
functions into a new file that now is built as a loadable module or
built-in, whichever is correct for a particular configuration. It fixes
the regression as well as the runtime problem that already existed.

The new module dependency chain is now:

   dvb-usb-{dibusb_mc,a800,dib0700,umt-010,gp8psk}   dvb-usb-dibusb-mb
         |                        |                   |          |
   dvb-usb-dibusb-mc-common       |        ___________|          |
         |               |        |        |                     |
   dib3000mc (frontend)  |        |        |         dib3000mb (frontend)
                         |        |        |
                         |        |        |
                        dvb-usb-dibusb-common

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>