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: NPeng Fan <van.freenix@gmail.com>
Acked-by: NAdrian Hunter <adrian.hunter@intel.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>