dm: mmc: sandbox: Update SD card emulation

The SDcard initialization procedure does a few more things than it did earlier:
* switch the bus width even for 1-bit bus width
* check that speed has been properly set (in resp[4] of SD_CMD_SWITCH_FUNC)

Update the SD simulator to handle those requests gracefully.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>