ALSA: pcsp: Use managed buffer allocation

Clean up the driver with the new managed buffer allocation API.
The superfluous snd_pcm_lib_malloc_pages() and
snd_pcm_lib_free_pages() are dropped.

Link: https://lore.kernel.org/r/20191209094943.14984-8-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>