ALSA: memalloc: Add fall-through annotation

As a preparatory patch for the upcoming -Wimplicit-fallthrough
compiler checks, add the "fall through" annotation in
snd_dma_alloc_pages().  Note that this seems necessary to be put
exactly before the next label, so it's outside the ifdef block.

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