mtk-sd: Prevent memory corruption from DMA map failure

If msdc_prepare_data() fails to map the DMA region, the request is
not prepared for data receiving, but msdc_start_data() proceeds
the DMA with previous setting.
Since this will lead a memory corruption, we have to stop the
request operation soon after the msdc_prepare_data() fails to
prepare it.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Fixes: 20848903 ("mmc: mediatek: Add Mediatek MMC driver")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/174972756982.3337526.6755001617701603082.stgit@mhiramat.tok.corp.google.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>