isdn/gigaset: encode HLC and BC together

Adapt to buggy device firmware which accepts setting HLC only in the
same command line as BC, by encoding HLC and BC in a single command
if both are specified, and rejecting HLC without BC.

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