mmc: xenon_sdhci: remove wait_dat0 SDHCI OP

Generic SDHCI driver received support for checking the busy status by
polling the DAT[0] level instead of waiting for the worst MMC switch time.

Unfortunately, it appears that this does not work for Xenon controllers
despite being a part of the standard SDHCI registers and the Armada 3720
datasheet itself telling that BIT(20) is useful for detecting the DAT[0]
busy signal.

I have tried increasing the timeout value, but I have newer managed to
catch DAT_LEVEL bits change from 0 at all.

This issue appears to hit most if not all SoC-s supported by Xenon driver,
at least A3720, A8040 and CN9130 have non working eMMC currently.

So, until a better solution is found drop the wait_dat0 OP for Xenon.
I was able to only test it on A3720, but it should work for others as well.

Fixes: 40e6f524 ("drivers: mmc: Add wait_dat0 support for sdhci driver")
Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Stefan Roese <sr@denx.de>