[ALSA] typo-fix and snd_assert()-expression-split

ALSA Core
This patch corrects a typo in the kerneldocs of snd_info_get_str().
It also splits the expressions of snd_assert() in snd_info_unregister()
into one-expression-per-call for better debugging.

Signed-off-by: Henrik Kretzschmar <henne@nachtwindheim.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>