mmc: core: fix the signaling 1.8V for HS200

Currently only 1.2V is treated for HS200 mode. If the host has only
1.8V I/O capability not 1.2V, mmc_set_signal_voltage can't be called
for 1.8V HS200. EXT_CSD_CARD_TYPE_SDR_1_8V needs to be considered.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Reviewed-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Chris Ball <cjb@laptop.org>