Revert "Revert "mmc: core: Refactor bus speed selection code""

This reverts commit 685de8253b902d8dfb6674e862b6745b42fcf4fb.

Change-Id: Ic1b83598984d59b406e287b79bc5065f3f5c2443
Merge "Revert "Revert "mmc: core: Refactor bus speed selection code""" into kk_2.7-stable