Fixed variable dereference issue in DDMA completion flow.

Fixes: b258e426 ("usb: dwc2: host: Fix ISOC flow in DDMA mode")
CC: stable@vger.kernel.org
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/linux-usb/2024040834-ethically-rumble-701f@gregkh/T/#m4c4b83bef0ebb4b67fe2e0a7d6466cbb6f416e39Signed-off-by: Minas Harutyunyan <Minas.Harutyunyan@synopsys.com>
Link: https://lore.kernel.org/r/cc826d3ef53c934d8e6d98870f17f3cdc3d2755d.1712665387.git.Minas.Harutyunyan@synopsys.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>