USB: ldusb: refactor endpoint retrieval

Use the new endpoint helpers to lookup the required interrupt-in
endpoint and optional interrupt-out endpoint.

Note that the descriptors are searched in reverse order to avoid any
regressions.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>