usb: dwc2: gadget: initialize controller in pullup callback

USB reset interrupt is no more used to reset the controller.
Thus, reset the controller in pullup callback as described by
Synopsys programming guide. Otherwise enumeration sometimes
fails when usb configuration is switched without physical
disconnection.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>