mmc: cavium: Fix probing race with regulator

If the regulator probing is not yet finished this driver
might catch a -EPROBE_DEFER. Returning after this condition
did not remove the created platform device. On a repeated
call to the probe function the of_platform_device_create
fails.

Calling of_platform_device_destroy after EPROBE_DEFER resolves
this bug.

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