mmc: core: Fix sequence for I/O voltage in DDR mode for eMMC

Even (e)MMC card can support 3.3v to 1.2v vccq in DDR, but not all
host controller can support this, like some of the SDHCI host
which connect to an eMMC device. Some of these host controller
still needs to use 1.8v vccq for supporting DDR mode.

So the sequence will be:
if (host and device can both support 1.2v IO)
	use 1.2v IO;
else if (host and device can both support 1.8v IO)
	use 1.8v IO;
so if host and device can only support 3.3v IO, this is the last choice.

Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Yunpeng Gao <yunpeng.gao@intel.com>
Tested-by: Jean-Michel Hautbois <jhautbois@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>