Fix a possible Oops in the Siemens Gigaset base driver when the device is
unplugged while an ISDN connection is still active, and makes sure that the
isdn4linux link level (LL) is properly informed if a connection is broken
by the USB cable being unplugged.

- Avoid unsafe checks of URB status fields outside the URB completion
  handlers, keep track of in-use URBs myself instead.

- If an isochronous transfer URB completes with status==0, also check the
  status of the frame descriptors.

- Verify length of interrupt messages received from the device.

- Align the length limit on transmitted AT commands with the device
  documentation.

- In case of AT response receive overrun, keep newly arrived instead of old
  unread data.

- Remove redundant check of device ID in the USB probe function.

- Correct and improve some comments and formatting.
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Acked-by: Hansjoerg Lipp <hjlipp@web.de>
Cc: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>