mmc: sdhci-pci: Move a function to avoid later forward declaration

Move a function to avoid having to forward declare it in a subsequent
patch.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Ludovic Desroches <ludovic.desroches@microchip.com>