bas_gigaset: suspend support

Add basic suspend/resume support to the bas_gigaset ISDN driver for the
Siemens Gigaset SX255 series of ISDN DECT bases.

Only the USB aspects are handled so far; the ISDN subsystem is not notified in
any way, for lack of information about how to do that.  The driver will refuse
to suspend if a connection is active.

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>