feat(st-sdmmc2): manage CMD6

For SD-cards, CMD6 is used to switch functions, like setting high speed
mode. As it has another meaning for eMMC, and may not work on standard
capacity SD-cards, it must be checked with MMC_IS_SD_HC flag.
As ACMD6 is also used, and will have the same index, a check on
CMD/ACMD commands is done: a boolean is stored depending on previous
command. It is set to true if CMD55 is issued, for other commands
it is set to false.

Change-Id: I6c2b9c7637656f858601ec075de1cb5f57af271a
Signed-off-by: Yann Gautier <yann.gautier@st.com>