[ALSA] race between disconnect and error handling in usbmidi

The driver resubmits URBs from an error handler and schedules the error
handler from the URBs' completion handlers. To reliably kill the cycle
a flag must be used.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>