usb: cdns3: gadget: initialize link_trb as NULL

There is an uninitialized variable "link_trb" usage at function cdns3_ep_run_transfer.
Fixed it by initialize "link_trb" as NULL.

Fixes: 4e218882 ("usb: cdns3: gadget: improve the dump TRB operation at cdns3_ep_run_transfer")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>