USB: serial: io_edgeport: fix epic endpoint lookup

BugLink: https://bugs.launchpad.net/bugs/1858489

commit 7c5a2df3 upstream.

Make sure to use the current alternate setting when looking up the
endpoints on epic devices to avoid binding to an invalid interface.

Failing to do so could cause the driver to misbehave or trigger a WARN()
in usb_submit_urb() that kernels with panic_on_warn set would choke on.

Fixes: 6e8cf775 ("USB: add EPIC support to the io_edgeport driver")
Cc: stable <stable@vger.kernel.org>     # 2.6.21
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191210112601.3561-5-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/usb/serial/io_edgeport.c

@@ -2859,16 +2859,18 @@ static int edge_startup(struct usb_serial *serial)
 	response = 0;
 
 	if (edge_serial->is_epic) {
+		struct usb_host_interface *alt;
+
+		alt = serial->interface->cur_altsetting;
+
 		/* EPIC thing, set up our interrupt polling now and our read
 		 * urb, so that the device knows it really is connected. */
 		interrupt_in_found = bulk_in_found = bulk_out_found = false;
-		for (i = 0; i < serial->interface->altsetting[0]
-						.desc.bNumEndpoints; ++i) {
+		for (i = 0; i < alt->desc.bNumEndpoints; ++i) {
 			struct usb_endpoint_descriptor *endpoint;
 			int buffer_size;
 
-			endpoint = &serial->interface->altsetting[0].
-							endpoint[i].desc;
+			endpoint = &alt->endpoint[i].desc;
 			buffer_size = usb_endpoint_maxp(endpoint);
 			if (!interrupt_in_found &&
 			    (usb_endpoint_is_int_in(endpoint))) {