mmc: mxcmmc: catch all errors when getting regulators

Bail out everytime when mmc_regulator_get_supply() returns an errno, not
only when probing gets deferred. This is currently a no-op, because this
function only returns -EPROBE_DEFER or 0 right now. But if it will throw
another error somewhen, it will be for a reason. (This still doesn't change
that getting regulators is optional, so 0 can still mean no regulators
found). So, let us a) be future proof and b) have driver code which is
easier to understand.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>