mmc: sdhci check parameters before call dma_free_coherent

We should not call dma_free_coherent if host->adma_table is NULL,
otherwise may trigger panic.

Fixes: d1e49f77

 ("mmc: sdhci: convert ADMA descriptors to a...")
Signed-off-by: Peng Fan <van.freenix@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>