usb: gadget: at91_udc: call at91udc_of_init only when needed

This avoids a build error in at91sam9261_9g10_defconfig:

drivers/usb/gadget/at91_udc.c: In function 'at91udc_probe':
drivers/usb/gadget/at91_udc.c:1685:34: warning: 'flags' may be used uninitialized in this
function [-Wmaybe-uninitialized]
  board->vbus_active_low = (flags & OF_GPIO_ACTIVE_LOW) ? 1 : 0;
                                  ^
drivers/usb/gadget/at91_udc.c:1678:21: note: 'flags' was declared here
  enum of_gpio_flags flags;
                     ^

Making the call to at91udc_of_init conditinal also reduces
the object code size without sacrificing build coverage.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>