ALSA: memalloc: Drop special handling of GFP for CONTINUOUS allocation

Now that all users of snd_dma_continuous_data() is gone, let's drop
this ugly (and dangerous) way.

After this commit, SNDRV_DMA_TYPE_CONTINUOUS may take the standard
device pointer instead of the hacked pointer by the macro above, and
the memalloc core refers to the coherent_dma_mask of the given
device like other SNDRV_DMA_TYPE.  It's still allowed to pass NULL
there, and in that case, the allocation is performed always in the
normal zone.

For SNDRV_DMA_TYPE_VMALLOC, the device pointer is simply ignored.

Link: https://lore.kernel.org/r/20220823115740.14123-5-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>