mmc: sdhci-cadence: do not use hardware tuning for SD mode

As commit ef6b7567

 ("mmc: sdhci-cadence: send tune request twice to
work around errata") stated, this IP has an errata. This commit applies
the second workaround for the SD mode.

Due to the errata, it is not possible to use the hardware tuning provided
by SDHCI_HOST_CONTROL2.

Use the software-controlled tuning like the eMMC mode.

Set sdhci_host_ops::platform_execute_tuning instead of overriding
mmc_host_ops::execute_tuning.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Link: https://lore.kernel.org/r/20200720061141.172944-1-yamada.masahiro@socionext.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>