usb: dwc3: gadget: introduce and use enable/disable irq methods

we don't need to enable IRQs until we have
a gadget driver loaded and ready to work,
so let's delay IRQ enable to ->udc_start()
and IRQ disable to ->udc_stop().

While at that, also move the related use of
request_irq() and free_irq().

Tested-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>