spi: Clear dma_mask for xilinx spi controller

mmc_spi driver tests if dma is available.
Microblaze systems supports DMA but xilinx_spi
controller doesn't support DMA transfers that's
why it is necessary to do this workaround to
clear dma_mask in spi controller.

mmc_spi then checks spi->master->dev.parent->dma_mask
which is cleared and dma mode is not used.

Signed-off-by: Michal Simek <monstr@monstr.eu>