mmc: sdhci: fix vmmc handling

Presently the vmmc regulator is enabled when the host
controller is added and disabled when it is removed.
However, the vmmc regulator should be under the control
of the upper layers via ->set_ios().  Make it so.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>