mmc: mxcmmc: update the regulator support code to the latest API

This also fixes the build problem introduced by my previous patch
due to unhandled API changes introduced by commit:
 99fc513101 (mmc: Move regulator handling closer to core)

Signed-off-by: Alberto Panizzo <maramaopercheseimorto@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>