usb: gadget: fix ptr_ret.cocci warnings

drivers/usb/gadget/udc/r8a66597-udc.c:1849:1-3: WARNING: PTR_ERR_OR_ZERO can be used

 Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>