gigaset: code cleanups

Some cleanups to the bas-gigaset and usb-gigaset USB ISDN drivers:
- simplified error handling
- improved debug messages
- readability improvements
- removal of obsolete defines and comments

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Cc: Greg KH <gregkh@suse.de>
Cc: Hansjoerg Lipp <hjlipp@web.de>
Cc: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>