mmc: renesas_sdhi: keep SCC clock active when tuning

mmc: renesas_sdhi: keep SCC clock active when tuning

Tuning procedure switches to lower frequencies but that will turn the
SCC off and accessing its register then will hang. So, check when we are
tuning and keep the current setup of the external clock if we are doing
so. Note that we still switch to the lower frequency because of the
internal divider. We just make sure to not modify the external clock.
This patch depends on a MMC core patch calling the downgrade function
earlier.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20200901150250.26236-4-wsa+renesas@sang-engineering.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>