spi: davinci: do not use DMA for vmalloc'ed buffers

Using vmalloc'ed buffers will fail since daVinci has
VIVT cache and only the kernel lowmem virtual address
is invalidated/flushed when performing DMA.
The virtual address returned from vmalloc() is not
invalidated/flushed and may contain stale data when
returning from spi_sync().
Fixes errors when running UBIFS over SPI NOR.
Revert this when all upper layer users of vmalloc'ed
buffers sent to SPI handles cache flushing/invalidating.

Signed-off-by: Frode Isaksen <fisaksen@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>