ALSA: usx2y: Use snd_card_free_when_closed() at disconnection

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

ALSA: usx2y: Use snd_card_free_when_closed() at disconnection

The USB disconnect callback is supposed to be short and not too-long

waiting. OTOH, the current code uses snd_card_free() at

disconnection, but this waits for the close of all used fds, hence it

can take long. It eventually blocks the upper layer USB ioctls, which

may trigger a soft lockup.

An easy workaround is to replace snd_card_free() with

snd_card_free_when_closed(). This variant returns immediately while

the release of resources is done asynchronously by the card device

release at the last close.