wusb: wusb-cbaf (CBA driver) sysfs ABI simplification

Simplify the sysfs ABI of the wusb-cbaf (Cable Based Association)
driver: use one value per file and cause the write of the CHID to
fetch the CDID (instead of requiring a separate read).

Update the example wusb-cbaf script to work with this revised ABI.

Signed-off-by: Felipe Zimmerle <felipe.zimmerle@indt.org.br>
Signed-off-by: David Vrabel <david.vrabel@csr.com>