mmc: sh_mobile_sdhi: improve prerequisites for tuning

Prerequisites for tuning are the same as for hw_reset. We need an SCC
and a supported mode. Populate the tuning related functions only when
those conditions are met. This also removes a tiny race window.
Previously, the functions were populated when the SCC offset was not
initialized which could have led to an OOPS.

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