ALSA: vx: Use managed buffer allocation

Clean up the driver with the new managed buffer allocation API.
The hw_params and hw_free callbacks became superfluous and got
dropped.

Link: https://lore.kernel.org/r/20191209094943.14984-9-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>