usb: gadget: udc: net2280: Improve patching of defect 7374

Once the defect 7374 is patched, there is no reason the keep reading the
idx scratch register.

Cache the content of the scratch idx register on device flag.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>