ALSA: memalloc: Minor refactoring

Return the pointer directly from alloc ops instead of setting
dmab->area at each place.  It simplifies the code a bit.

Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210802072815.13551-2-tiwai@suse.de


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