USB: opticon: simplify bulk-in discovery in attach

Remove custom end-point iteration which has already been taken care of
by usb-serial core.

The first bulk-in endpoint found will be associated with the first port.

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