ALSA: doc: Drop snd_dma_continuous_data() usages

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.

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