Revert "mmc: meson-gx: include tx phase in the tuning process"

This reverts commit 0a446976.

This commit was initially intended to fix problems with hs200 and hs400
on some boards, mainly the odroid-c2. The OC2 (Rev 0.2) I have performs
well in this modes, so I could not confirm these issues.

We've had several reports about the issues being still present on (some)
OC2, so apparently, this change does not do what it was supposed to do.
Maybe the eMMC signal quality is on the edge on the board. This may
explain the variability we see in term of stability, but this is just a
guess. Lowering the max_frequency to 100Mhz seems to do trick for those
affected by the issue

Worse, the commit created new issues (CRC errors and hangs) on other
boards, such as the kvim 1 and 2, the p200 or the libretech-cc.

According to amlogic, the Tx phase should not be tuned and left in its
default configuration, so it is best to just revert the commit.

Fixes: 0a446976

 ("mmc: meson-gx: include tx phase in the tuning process")
Cc: <stable@vger.kernel.org> # 4.14+
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>