mmc: mmc_spi: add checks for dma mapping error

There is no checks for dma mapping errors in mmc_spi.
Tha patch fixes that and by the way it adds dma_unmap_single(ones_dma)
that was left on a failure path mmc_spi_probe().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>