ALSA: usb-audio: Avoid potential endless loop in convert_chmap_v3()

In the Linux kernel, the following vulnerability has been resolved: ALSA: usb-audio: Avoid potential endless loop in convert_chmap_v3() The convert_chmap_v3() has a loop with its increment size of cs_desc->wLength, but we forgot to validate cs_desc->wLength itself, which may lead to potential endless loop by a malformed descriptor. Add a proper size check to abort the loop for plugging the hole.