mmc: core: Factor out common code in drive strength selection

Make a new function out of common code used for drive
strength selection.

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

diff --cc drivers/mmc/core/core.c
Simple merge

diff --cc drivers/mmc/core/core.h
Simple merge

diff --cc drivers/mmc/core/sd.c
Simple merge

diff --cc drivers/mmc/core/sdio.c
Simple merge