usb: dwc3: Do core validation early on probe

The setting of the dr_mode may need to check the controller's revision.
The revision is set in the dwc3_core_is_valid(), which comes after
dr_mode setting. Let's move it closer to the start of the dwc3_probe()
function and before calling dwc3_get_dr_mode().

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>