mmc: renesas_sdhi: only reset SCC when its pointer is populated

Only re-initialize SCC and tuning when an SCC was found during probe().
This is currently a noop because all R-Car Gen2+ are considered to have
an SCC. But this will change in a later patch, so we need this
preparation.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20201110142058.36393-2-wsa+renesas@sang-engineering.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>