mmc: dw_mmc: Handle late vmmc regulators with EPROBE_DEFER

It is possible to specify a regulator that should be turned on when
dw_mmc is probed.  At the moment dw_mmc will fail to use the regulator
properly if the regulator probes after dw_mmc.  Fix this problem by
honoring EPROBE_DEFER.

At the same time move the regulator code out of the slot init code.
We only specify one regulator for the whole device and other parts of
the code (like suspend/resume) assume that the regulator has only been
enabled once.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Chris Ball <cjb@laptop.org>