usb: dwc3: Fix error handling for core init

Fixing the sequence of events in dwc3_core_init() error exit path.
dwc3_core_exit() call is also removed from the error path since,
whatever it's doing is already done.

Fixes: c499ff71 usb: dwc3: core: re-factor init and exit paths

Cc: Felipe Balbi <felipe.balbi@linux.intel.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Stable <stable@vger.kernel.org> # 4.8+
Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>