ALSA: usb - Fix possible Oops at USB-MIDI disconnection

The endpoints should be released immediately at disconnection
rather than the delayed release.  This could be a reason of Oops
at USB-audio device disconnection being used.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>