isdn: move capi drivers to staging

I tried to find any indication of whether the capi drivers are still in
use, and have not found anything from a long time ago.

With public ISDN networks almost completely shut down over the past 12
months, there is very little you can actually do with this hardware. The
main remaining use case would be to connect ISDN voice phones to an
in-house installation with Asterisk or LCR, but anyone trying this in
turn seems to be using either the mISDN driver stack, or out-of-tree
drivers from the hardware vendors.

I may of course have missed something, so I would suggest moving these
three drivers (avm, hysdn, gigaset) into drivers/staging/ just in case
someone still uses them.

If nobody complains, we can remove them entirely in six months, or
otherwise move the core code and any drivers that are still needed back
into drivers/isdn.

As Paul Bolle notes, he is still testing the gigaset driver as long as
he can, but the Dutch ISDN network will be shut down in September 2019,
which puts an end to that.

Marcel Holtmann still maintains the Bluetooth CMTP profile and wants to
keep that alive, so the actual CAPI subsystem code remains in place for
now, after all other drivers are gone, CMTP and CAPI can be merged into
a single driver directory.

Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>