ALSA: usb-audio: Fix potential memory leaks

In the Linux kernel, the following vulnerability has been resolved:

ALSA: usb-audio: Fix potential memory leaks

When the driver hits -ENOMEM at allocating a URB or a buffer, it

aborts and goes to the error path that releases the all previously

allocated resources. However, when -ENOMEM hits at the middle of the

sync EP URB allocation loop, the partially allocated URBs might be

left without released, because ep->nurbs is still zero at that point.

Fix it by setting ep->nurbs at first, so that the error handler loops

over the full URB list.