ASoC: wm_adsp: Use vmalloc to allocate firmware download buffer

Use vmalloc to allocate the buffer for firmware/coefficient download and
rely on the SPI core to split this up into DMA-able chunks. This should
give better performance and means we no longer need to manually split
the download into page size chunks to avoid allocating overly large
continuous memory regions.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>