mmc: block: Always switch back to main area after RPMB access

In preparation to support the use of the RPMB partition with transfer
modes that might require re-tuning, always switch back to the main
area after RPMB access.

RPMB is accessible only via IOCTL so only those paths are affected.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>