ALSA: firewire-lib: suppress kernel warnings when releasing uninitialized stream data

When any of AMDTP stream data are not initialized and private data is
going to be released, WARN_ON() in amdtp_stream_destroy() is hit and
dump messages. This may take users irritated.

This commit fixes the bug to skip releasing when it's not initialized.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>