mmc: mtk-sd: enable internal write-protect logic.

The mtk-sd silicon has integrated write-protect detection logic that is
enabled, at least, on the MT7621 as used in the GNUBEE NAS.

If the sdhci isn't marked no-write-protect and doesn't have a
ro-gpio configured, assume the internal wp logic should be used.

Signed-off-by: NeilBrown <[email protected]>

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