ARM: OMAP2: fix musb usage for n8x0

Commit b7e2e75a ("usb: gadget: drop unused USB_GADGET_MUSB_HDRC")
dropped a config symbol that was unused by the musb core, but it turns
out that board support code still had references to it.

As the core now handles both dual role and host-only modes, we can just
pass MUSB_OTG as mode from board files.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Olof Johansson <olof@lixom.net>