USB: race fixes for usb-serial step 1

- introduce a spinlock for serial_table to eliminate the window between
  looking up a device and getting a reference
- delay inscription of a new device into serial_table until it is fully
  initialised
- make sure disconnect() kills all URBs to avoid leckage across a soft unbind

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>