mmc: core: Allow invalid regulator in mmc_regulator_set_ocr()

Basically all host drivers use code like this:

if (!IS_ERR(mmc->supply.vmmc))
	mmc_regulator_set_ocr(mmc, mmc->supply.vmmc, 0)

Move the IS_ERR() check to mmc_regulator_set_ocr() to simplify host driver
code.

Suggested-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/0c1e7440-f4bc-b48a-137f-58b75ac98550@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>