mmc: sdhci: move sdhci_pre_dma_transfer()

Move sdhci_pre_dma_transfer() to avoid needing to declare this function
before use.

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>