USB: option: revert introduction of struct option_private

This is a partial, context modified revert of commit e463c6dd
("USB: option: handle send_setup blacklisting at probe"), which
introduced an unnecessary struct option_private for storing the
interface number used in option_send_setup. Removing this struct
will allow option_send_setup to be generalized for other drivers.

Suggested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: Johan Hovold <johan@kernel.org>