mmc: sdhci: Export sdhci_request()

Allow SDHCI drivers to hook code before and after sdhci_request() by
making it externally visible.

Signed-off-by: Aapo Vienamo <avienamo@nvidia.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>