gigaset: stop maintaining seperately

The Dutch consumer grade ISDN network will be shut down on September 1,
2019. This means I'll be converted to some sort of VOIP shortly. At that
point it would be unwise to try to maintain the gigaset driver, even for
odd fixes as I do. So I'll stop maintaining it as a seperate driver and
bump support to CAPI in staging. De facto this means the driver will be
unmaintained, since no-one seems to be working on CAPI.

I've lighty tested the hardware specific modules of this driver (bas-gigaset,
ser-gigaset, and usb-gigaset) for v5.3-rc1. The basic functionality appears to
be working. It's unclear whether anyone still cares. I'm aware of only one
person sort of using the driver a few years ago.

Thanks to Karsten Keil for the ISDN subsystems gigaset was using (I4L and
CAPI). And many thanks to Hansjoerg Lipp and Tilman Schmidt for writing and
upstreaming this driver.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>