USB: serial: metro-usb: add missing interrupt-out endpoint check

One class of "unidirectional" devices managed by this driver uses an
interrupt-out endpoint to send control messages at open and close. Due
to a missing endpoint sanity check, this could result in an interrupt
URB being submitted to endpoint 0 instead. This would be caught by
USB core (without a WARN dump), but let's verify that the expected
endpoints are present at probe rather than when a port is later opened.

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