isdn/gigaset: use gig_dbg() for debugging output

The "TTY buffer in tty_port" patchset introduced an opencoded
debug message in the Gigaset tty device if_close() function.
Change it to use the gig_dbg() macro like everywhere else in
the driver.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>