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

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

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

fcp_init_notify() used mixer->urb, which snd_usb_mixer_status_create()
allocates for the optional UAC2 status interrupt endpoint and mixer.c
kills, resubmits and frees. On a device with that endpoint,
fcp_init_notify()'s "already set up" early return fires on the status
URB and returns success without doing anything. No FCP notification
URB is submitted, and cmd_done is left zeroed because it is
initialised past that early return and nowhere else. fcp_init() then
issues init1_opcode and wait_for_completion_timeout() would crash
adding to the zeroed wait.head. fcp_cleanup_urb() would also kill and
free mixer.c's status URB.

Use a separate URB in fcp_data, and initialise cmd_done in
fcp_init_private() where fcp_data is allocated. fcp_init_notify() is
reached again after suspend via fcp_reinit(), and the URB kill path in
fcp_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.