usb: dwc3: gadget: Fix ISO transfer performance

Commit 08a36b54 ("usb: dwc3: gadget: simplify __dwc3_gadget_ep_queue()")
caused a small change in the way ISO transfer is handled in the case
when XferInProgress event happens on Isoc EP with an active transfer.
This caused a performance degradation of 50%. e.g. using g_webcam on DUT
and luvcview on host the video frame rate dropped from 16fps to 8fps
@high-speed.

Make the ISO transfer handling equivalent to that prior to that commit
to get back the original ISO performance numbers.

Fixes: 08a36b54 ("usb: dwc3: gadget: simplify __dwc3_gadget_ep_queue()")
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>