[ALSA] cleanup and error reporting for sound/core/init.c

Make the control flow clear with indentation, adds some comments
and improves error reporting.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>