mmc: tmio: don't wait on R-Car2+ when handling the clock

Our hardware engineers confirmed that it is unnecessary to wait when
turning the clock on/off. The documentation was a tad vague, so we
used to play safe.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>