mmc: core: mmc DDR mode should not depend on UHS_DDR50
The MMC_CAP_UHS_DDR50 must work on 1.8v.
However, the eMMC DDR mode can work on either 1.8v or 3.3v and
should not depend on UHS_DDR50.
So get rid of this limitation to let controller without 1.8v
signal voltage support can also work for eMMC DDR mode if it claims.