usb: gadget: udc: at91: use PTR_ERR_OR_ZERO()

coccicheck found this pattern which could be
converted to PTR_ERR_OR_ZERO(). No functional
changes.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>