USB: cdc-wdm: make module autoload work

this fixes an omission that led to no alias being computed for the
cdc-wdm module.

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