usb: gadget: aspeed_udc: fix handling of tx_len == 0

The bug is that we should still enter this loop if "tx_len" is zero.

After adding the "last" variable, then the "chunk >= 0" condition is no
longer required but I left it for readability.

Fixes: c09b1f37 ("usb: gadget: aspeed_udc: cleanup loop in ast_dma_descriptor_setup()")
Reported-by: Neal Liu <neal_liu@aspeedtech.com>
Reviewed-by: Neal Liu <neal_liu@aspeedtech.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/Yrq6F5okoX1y05rT@kiliSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/aspeed_udc.c

@@ -476,6 +476,7 @@ static int ast_dma_descriptor_setup(struct ast_udc_ep *ep, u32 dma_buf,
 {
 	struct ast_udc_dev *udc = ep->udc;
 	struct device *dev = &udc->pdev->dev;
+	bool last = false;
 	int chunk, count;
 	u32 offset;
 
@@ -493,14 +494,16 @@ static int ast_dma_descriptor_setup(struct ast_udc_ep *ep, u32 dma_buf,
 	       "tx_len", tx_len);
 
 	/* Create Descriptor Lists */
-	while (chunk > 0 && count < AST_UDC_DESCS_COUNT) {
+	while (chunk >= 0 && !last && count < AST_UDC_DESCS_COUNT) {
 
 		ep->descs[ep->descs_wptr].des_0 = dma_buf + offset;
 
-		if (chunk > ep->chunk_max)
+		if (chunk > ep->chunk_max) {
 			ep->descs[ep->descs_wptr].des_1 = ep->chunk_max;
-		else
+		} else {
 			ep->descs[ep->descs_wptr].des_1 = chunk;
+			last = true;
+		}
 
 		chunk -= ep->chunk_max;