usb: sisusb: let files build only when needed

After the previous patch we see, that whole files are ifdeffed depending
on CONFIG options. So do not build the files at all if the CONFIG is not
enabled. (I.e. move the check from .c to Makefile.)

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>