gigaset: Kconfig cleanup

Streamline dependencies and remove some obsolete or redundant comments
in the Gigaset ISDN driver's Kconfig file. In particular, remove the
strong warning against the GIGASET_UNDOCREQ option, as in seven years
of existence, the code in question has never been reported to cause
any harm.

Impact: Kconfig cleanup, no functional change

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>