ALSA: es1688: Add the missed snd_card_free()

snd_es968_pnp_detect() misses a snd_card_free() in a failed path.
Add the missed function call to fix it.

Fixes: a20971b2

 ("ALSA: Merge es1688 and es968 drivers")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200603092459.1424093-1-hslester96@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>