usb: ohci-da8xx can only be built-in

The PHY setup code of the TI DaVinci DA8xx OHCI controller
uses ad-hoc register access using a pointer that is meant to
be used only by the DaVinci platform implementation and that
is intentionally not exported to loadable modules. This results
in a link error on configurations that use a modular OHCI
code on this platform.

While the proper solution for this problem would be to
implement a real PHY driver shared by ohci-da8xx and musb-da8xx,
this patch for now just works around the build error by
only allowing the ohci-da8xx code to be built-in.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>