mmc: sdhci: cleanup DMA un-mapping

The patch "mmc: sdhci: plug DMA mapping leak on error" added
un-mapping logic to sdhci_tasklet_finish() where it is always
called, thereby preventing the mapping leaking.

Consequently the un-mapping code in sdhci_finish_data() is no
longer needed.  Remove it.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
[ Split from original "mmc: sdhci: plug DMA mapping leak on error" patch ]
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>