ALSA: memalloc: Fix missing return value comments for kernel docs

Each kernel doc comment expects the definition of the return value in
a proper format.  This patch adds or fixes the missing entries for
memory allocation helpers.

Link: https://lore.kernel.org/r/20220713104759.4365-7-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>