USB: race fixes for usb-serial, step 2

- take BKL before looking up a driver to associate with a device to make
  sure the module is not unloaded after looking up but before association
  & bumping module count

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