usb: gadget: atmel_usba: always test udc->driver

Found using smatch: drivers/usb/gadget/atmel_usba_udc.c:1689 usba_udc_irq()
error: we previously assumed 'udc->driver' could be null (see line 1636)

Always test udc->driver before using its members.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>