mmc: usdhi6rol0: handle probe deferral for regulator

We ignore errors from mmc_regulator_get_supply() because the usage of
the regulators is optional for the driver, but we still need to check
for and handle EPROBE_DEFER, like it's done in for example dw_mmc.
Otherwise we might end up not using the specified regulators just
because of probe order.

Signed-off-by: Rabin Vincent <rabin.vincent@axis.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>