usb: gadget: udc: net2280: Simplify scan_dma_completions

After fix superspeed dma_done was applied we can simplify the code by
removing the duplicated dma_done and letting the function check if there
are more completed dma transactions.

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