ALSA: scarlett2: Use a private URB for the notification endpoint

Summary

In the Linux kernel, the following vulnerability has been resolved: ALSA: scarlett2: Use a private URB for the notification endpoint scarlett2_init_notify() used mixer->urb, which snd_usb_mixer_status_create() allocates for the UAC2 status interrupt endpoint and mixer.c manages. On a device with that endpoint, the "already in use" check fires on the status URB and returns 0 for success without doing anything. No notification URB is submitted, and cmd_done is left zeroed because it is initialised past that check and nowhere else. scarlett2_usb_init() then issues SCARLETT2_USB_INIT_1 and wait_for_completion_timeout() would crash adding to the zeroed wait.head. Use a separate URB in scarlett2_data, as done for FCP, and initialise cmd_done in scarlett2_init_private(). mixer.c was also freeing the URB in snd_usb_mixer_free() and resubmitting it in snd_usb_mixer_activate(), so scarlett2 must now do both: add scarlett2_cleanup_urb(), called from private_free and private_suspend, and a private_resume callback to re-establish the URB after resume. scarlett2_init_notify() is reached from there, and the URB kill path in scarlett2_notify() completes cmd_done, leaving a stale count that would satisfy the next command's wait before the device ACKs. Use reinit_completion() to clear it. Also free the URB if the transfer buffer allocation fails, and both if usb_submit_urb() fails. Move scarlett2_init_notify() up next to scarlett2_cleanup_urb() so scarlett2_init_private() can reference it without a forward declaration.