usb: dwc3: st: prepare the driver for generic usb_get_dr_mode function

of_usb_get_dr_mode will be converted into more generic
usb_get_dr_mode function that will take struct device
instead of struct device_node as its parameter.

To make the conversion possible later, waiting for the
platform device for dwc3 to be populated before calling
of_usb_get_dr_mode.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
CC: Giuseppe Cavallaro <peppe.cavallaro@st.com>
CC: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>