Update the documentation to follow the recent change of the memory
allocation helpers. The macro snd_dma_continuous_data() is gone, and
the driver needs to set up the coherent dma mask for allocating in the
lower memory addresses, instead.
Link: https://lore.kernel.org/r/20220823115740.14123-6-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
