ALSA: opl3sa2: drop snd_card pointer from the snd_opl3sa2 structure

Remove card pointer from the snd_opl3sa2
structure and break circular reference
snd_card->snd_opl3sa2->snd_card.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Acked-by: Rene Herman <rene.herman@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>