cdc-acm: fix wrong pipe type on rx interrupt xfers

This fixes the "BOGUS urb xfer" warning logged by usb_submit_urb().

Signed-off-by: Gavin Li <git@thegavinli.com>
Acked-by: Oliver Neukum <oneukum@suse.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>