usb: common: of_usb_get_dr_mode to usb_get_dr_mode

By using the unified device property interface, the function
can be made available for all platforms and not just the
ones using DT.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>