mmc: block: Pause re-tuning while switched to the RPMB partition

Re-tuning is not possible when switched to the RPMB
partition.  However re-tuning should not be needed
if re-tuning is done immediately before switching,
a small set of operations is done, and then we
immediately switch back to the main partition.

A previous patch ensured that we immediately switch
back to the main partition.

This patch uses the new facility to "pause" re-tuning
before switching to the RPMB partition, and to "unpause"
it after switching from the RPMB partition.

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