Use enum usb_dr_mode and drop default usb_dr_mode from platform data.

USB DT bindings states: dr_mode: "...In case this attribute isn't
passed via DT, USB DRD controllers should default to OTG...",
so remove redundand field.
Signed-off-by: Ivan T. Ivanov <iivanov@mm-sol.com>
Acked-by: David Brown <davidb@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>