usb: dwc3: gadget: Fix start transfer errors for ISOC endpoints

If the end transfer does not happen on ISOC endpoints
as a part of endpoint disable during set alt transitions;
the consequent start transfers are failed.
Fix this by explicitly doing stop active transfers on
ISOC endpoints during endpoint disable.

Change-Id: I66f8c7d10cfee4ba1c816ad0c1d2486f86936d34
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>