ALSA: firewire-tascam: fix memory leak of private data

Although private data of sound card instance is usually allocated in the
tail of the instance, drivers in ALSA firewire stack allocate the private
data before allocating the instance. In this case, the private data
should be released explicitly at .private_free callback of the instance.

This commit fixes memory leak following to the above design.

Fixes: b610386c

 ('ALSA: firewire-tascam: deleyed registration of sound card')
Cc: <stable@vger.kernel.org> # v4.7+
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>