usb: dwc3: gadget: issue Update Transfer command after queuing isoc request

Issue an Update Transfer command after queuing a request to an isoc
EP with an active transfer. This is required according to the dwc3
databook. Pratyush Anand reports that this fixes a problem he was
having with Isoc IN transfers.

Tested-by: Pratyush Anand<pratyush.anand@st.com>
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>