mmc: cavium: Prevent crash with incomplete DT

In case the DT specifies neither a regulator nor a gpio
for the shared power the driver will crash accessing the regulator.
Prevent the crash by checking the regulator before use.

Use mmc_regulator_get_supply() instead of open coding the same
logic.

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>