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>