usb: dwc3: adapt to use dr_mode device tree helper

This patch adapts the dwc3 to use the device tree helper
"of_usb_get_dr_mode" for the mode of operation of the dwc3 instance
being probed.

[ balbi@ti.com : make of_usb_get_dr_mode() conditional on
	dev->of_node and let pdata pass dr_mode too ]

Reviewed-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Ruchika Kharwar <ruchika@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>