omap: Devkit8000: Fix wrong usb port on Devkit8000

Disable the nonexistent ehci port on Devkit8000.

Signed-off-by: Thomas Weber <weber@corscience.de>
Acked-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>