ALSA: snd-usb: fix stream info output in /proc

Set some substream struct members to make the proc interface code work
again.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reported-by: Felix Homann <linuxaudio@showlabor.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>