usb: dwc2: Postponed gadget registration to the udc class driver

During dwc2 driver probe, after gadget registration to the udc class
driver, if exist any builtin function driver it immediately bound to
dwc2 and after init host side (dwc2_hcd_init()) stucked in host mode.
Patch postpone gadget registration after host side initialization done.

Fixes: 117777b2

 ("usb: dwc2: Move gadget probe function into platform code")
Reported-by: kbuild test robot <lkp@intel.com>
Tested-by: Marek Vasut <marex@denx.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Minas Harutyunyan <hminas@synopsys.com>
Link: https://lore.kernel.org/r/f21cb38fecc72a230b86155d94c7e60c9cb66f58.1591690938.git.hminas@synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>