usb: dwc3: ep0: use gadget->isoch_delay for isoch_delay value

Instead of keeping our own isoch_delay, let's make use of the newly
introduced isoch_delay member in struct usb_gadget. The benefit here
is that we would be using a generic "API" which other UDCs can use,
resulting in a common setup for gadget drivers who may be interested
in Isoch Delay value.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>