mmc: core: Improve support for deferred regulators

Callers of mmc_regulator_get_supply could benefit from knowing if either
of the regulators are present but not yet available.  Since callers do
not currently examine the return value, modify this function to return
zero or -EPROBE_DEFER if either regulator get returns the same.

Furthermore, since callers check vmmc/vqmmc using IS_ERR and can deal
with absent regulators, switch to devm_regulator_get_optional. This has
the added benefit of allowing this function to behave correctly even in
the !CONFIG_REGULATOR case such that the stub can be removed.

Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>