mmc: start removing enable / disable API

Most parts of the enable / disable API are no longer used and
can be removed.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Venkatraman S <svenkatr@ti.com>
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>