usb: Correct libusb timeout handling

The documentation for libusb_bulk_transfer() specifies that in the event
that the read or write request is split into multiple chunks, some of
these may still be transferred despite a LIBUSB_ERROR_TIMEOUT is
returned.

In the transition to libusb this detail was missed and completed
read transfers are sometimes considred to be timeouts and the data
discarded, obviously resulting in failure to continue.

Consider the "transferred" value in the event of timeout, to avoid this.

Although not yet spotted in testing, the same logic is introduced for
the write path.

Signed-off-by: Bjorn Andersson <[email protected]>

usb.c

@@ -198,7 +198,8 @@ int qdl_read(struct qdl_device *qdl, void *buf, size_t len, unsigned int timeout
 	int ret;
 
 	ret = libusb_bulk_transfer(qdl->usb_handle, qdl->in_ep, buf, len, &actual, timeout);
-	if (ret < 0)
+	if ((ret != 0 && ret != LIBUSB_ERROR_TIMEOUT) ||
+	    (ret == LIBUSB_ERROR_TIMEOUT && actual == 0))
 		return -1;
 
 	return actual;
@@ -218,7 +219,8 @@ int qdl_write(struct qdl_device *qdl, const void *buf, size_t len)
 
 		ret = libusb_bulk_transfer(qdl->usb_handle, qdl->out_ep, data,
 					   xfer, &actual, 1000);
-		if (ret < 0) {
+		if ((ret != 0 && ret != LIBUSB_ERROR_TIMEOUT) ||
+		    (ret == LIBUSB_ERROR_TIMEOUT && actual == 0)) {
 			warnx("bulk write failed: %s", libusb_strerror(ret));
 			return -1;
 		}