ALSA: core: Fix possible memory leaks at error path in info.c

Currently, snd_info_init() just returns an error without releasing the
previously assigned resources at error path.  The assigned proc and
info entries have to be released properly.  This patch covers it.

While we are at it, refactor the code a bit, too.

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