ALSA: Fix allocation size calculation in snd_dma_alloc_pages_fallback()

snd_dma_alloc_pages_fallback() always tries to reduce the size in a half,
but it's not good when the given size isn't a power-of-two.
Check it first then try to align.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>