ser_gigaset: convert mutex to completion

The ser_gigaset ISDN driver was using a mutex in its close() method for
waiting for other running ldisc methods to finish.  That's what completions
are for.  Incidentally, this also avoids a spurious "BUG: lock held at task
exit time" message when the driver's userspace daemon daemonizes itself.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>