USB: omninet: clean up protocol defines

Remove redundant data-offset define, which was really just the header
length.

Add payload-size define and use the bulk-out size define for the actual
bulk-out size.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>