mmc: sdhi: fill in actual_clock

Save set clock in mmc_host actual_clock enabling exporting it via debugfs.
This will indicate the precise SD clock in I/O settings rather than only the
sometimes misleading requested clock.

Signed-off-by: Tamás Szűcs <tszucs@protonmail.ch>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>