mtd: replace the hardcode with the onfi_feature()

The current code uses the hardcode to detect the 16-bit bus width.
Use the onfi_feature() to replace it.

Signed-off-by: Huang Shijie <b32955@freescale.com>
[Brian: small fixup]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>