mmc: correct the HS400 initialization process

mmc: correct the HS400 initialization process

After the commit b9a2a0e2e9c0 ("mmc: Add support for downgrading
HS200/HS400 to HS mode"), it add a parameter in mmc_set_card_speed()
which indicates that the HS200/HS400 to HS downgrade is happening.

During the HS400 initialization, first select to HS200, and config
the related clock rate, then downgrade to HS mode. So here also need
to config the downgrade value to be true for two reasons. First,
make sure in the function mmc_set_card_speed(), after switch to HS
mode, first config the clock rate, then read the EXT_CSD, avoid
receiving data of EXT_CSD in HS mode at 200MHz. Second, after issue
the MMC_CMD_SWITCH command, it need to wait a bit then switch bus
properties.

Test on i.MX8QM MEK board, some Micron eMMC will stuck in transfer
mode in this case, and USDHC will never get data transfer complete
status, cause the uboot hang.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>