spi: spi-pxa2xx: Remove unused legacy null dma buffer and allocation for it

Remove null_dma_buf variable and extra allocation for it. It is not needed
since commit 6356437e ("spi: spi-pxa2xx: remove legacy PXA DMA bits").

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>