usb: dwc3: core: Move the mode setting to the right place

When dwc3 core enters into suspend mode, the system (especially for mobile
device) may power off the dwc3 controller for power saving, that will cause
dwc3 controller lost the mode operation when resuming dwc3 core.

Thus we can move the mode setting into dwc3_core_init() function to avoid this
issue.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>