mmc: meson-gx: improve initial configuration

Config values which are not changed during runtime we can set
in the probe function already.
The block size setting is overwritten later in
meson_mmc_start_cmd anyway if needed, so it doesn't harm if we
remove this setting in meson_mmc_set_ios.

In addition write config register only if configuration changed.

Don't change the location of clock initialization as in an
earlier version of the patch, this change causes a hang.
This issue was reported and fix suggested by:
Helmut Klein <hgkr.klein@gmail.com>

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>