mmc: sdhci-esdhc-imx: separate transfer mode from command write for usdhc

The combining of SDHCI_TRANSFER_MODE and SDHCI_COMMAND writes is only
required for esdhc, but not necessarily for usdhc.  Different from
esdhc where the bits for transfer mode and command are all in the same
register CMD_XFR_TYP, usdhc has a newly introduced register MIX_CTRL
to hold transfer mode bits.  So it makes more sense to separate transfer
mode from command write for usdhc.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Chris Ball <cjb@laptop.org>