mmc: sdhci: disable the 8bit mode when host doesn't support it

Buswidth is depeneded on Hardware schematic.
Evne though host can support the 8bit buswidth, if hardware doesn't
support 8bit mode, it doesn't work fine.
So the buswidth mode selection leaves a matter in each SoC drivers.

On the contrary to this, hardware supports 8bit mode, but host doesn't
support it. then controller has to disable the MMC_MODE_8BIT.
(Host can check whether 8bit mode is supported or not, since V3.0)

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>