usb: dwc3: core: simplify suspend/resume operations

now that we have re-factored dwc3_core_init() and
dwc3_core_exit() we can use them for suspend/resume
operations.

This will help us avoid some common mistakes when
patching code when we have duplicated pieces of code
doing the same thing.

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