mmc: host: omap_hsmmc: use devm_regulator_get_optional() for vmmc

Since vmmc can be optional for some platforms, use
devm_regulator_get_optional() for vmmc. Now return error only
if the return value of devm_regulator_get_optional() is not the
same as -ENODEV, since with -EPROBE_DEFER, the regulator can be
obtained later and all other errors are fatal.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>