mmc: sdhci: always unmap a mapped data transfer in sdhci_post_req()

If the host cookie indicates that the data buffers of a request are
mapped at sdhci_post_req() time, always unmap the data buffers.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>