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

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: c09b1f372e74 ("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@kili
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>