usb: dwc3: core: re-factor init and exit paths

The idea of this patch is for dwc3_core_init() to
abstract all the details about how to initialize
dwc3 and dwc3_core_exit() to do the same for
teardown.

With this, we can simplify suspend/resume operations
by a large margin and always know that we're going
to start dwc3 from a known starting point.

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