ALSA: core: Fix missing card sysfs contents

While moving the card device into struct snd_card, the reference to
the assigned card in sysfs show/store callbacks were forgotten to be
refreshed, still accessing to the no longer used drvdata.  Fix these
places to refer correctly via container_of().

Also, remove the superfluous NULL checks since it's guaranteed to be
non-NULL now.

Fixes: 8bfb181c ('ALSA: Embed card device into struct snd_card')
Signed-off-by: Takashi Iwai <tiwai@suse.de>