mmc: core: Add new power_mode MMC_POWER_UNDEFINED

Add MMC_POWER_UNDEFINED for power_mode in struct mmc_ios and use it as
the initial value of host->ios.power_mode.

For hosts with MMC_CAP2_NO_PRESCAN_POWERUP, this makes the later
mmc_power_off() do real power-off things instead of NOP, and further
prevents state messed up in cards that was already initialized (eg. by
BIOS of UEFI driver).

Signed-off-by: Roger Tseng <rogerable@realtek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>