ALSA: opl3sa2 - Fix NULL dereference when suspending snd_opl3sa2

Fix the OOPS during a opl3sa2 card suspend
and resume if the driver is loaded but the card
is not found.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>