USB: Fix makefile so that CONFIG_WDM and CONFIG_TMC work.

If CONFIG_USB_ACM and CONFIG_USB_PRINTER are not set, then
cdc-wdm and usbtmc won't get built.

Signed-off-by: Andy Lutomirski <amluto@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>