ALSA: Fix uninintialized error return

Static analysis with cppcheck found the following error:
  [sound/core/init.c:118]: (error) Uninitialized variable: err

..this was introduced by commit 2471b6c8
("ALSA: info: Register proc entries recursively, too") where the call
to snd_info_card_register was removed and no longer setting the error
return in err.  When snd_info_create_card_entry fails to allocate a
an entry, the error path exits with garbage in err.  Fix is to return
-ENOMEM if entry fails to be allocated.

Fixes: 2471b6c8 ("ALSA: info: Register proc entries recursively, too")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>