mmc: sdhci: Always allow tuning to fall back to fixed sampling

SDHCI falls back to fixed sampling if there is an error during tuning.
However it also reports an error unless there is periodic re-tuning.
That is not the best option because:
a) there is a reasonable chance that fixed sampling will work, especially
at room temperature.
b) re-tuning will be done again anyway if there are CRC errors.
Change to return no error always when falling back to fixed sampling.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>