USB: gadget: Fix return of -EBUSY

Currently when driver->match_existing_only is true, the error return is
set to -EBUSY however ret is then set to 0 at the end of the if/else
statement. I believe the ret = 0 statement should be set in the else
part of the if statement and not at the end to ensure -EBUSY is being
returned correctly.

Detected by clang scan:
drivers/usb/gadget/udc/core.c:1558:4: warning: Value stored to 'ret' is
never read [deadcode.DeadStores]

Fixes: fc274c1e ("USB: gadget: Add a new bus for gadgets")
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20220504135840.232209-1-colin.i.king@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/core.c

@@ -1559,8 +1559,8 @@ int usb_gadget_register_driver_owner(struct usb_gadget_driver *driver,
 		} else {
 			pr_info("%s: couldn't find an available UDC\n",
 					driver->function);
+			ret = 0;
 		}
-		ret = 0;
 	}
 	mutex_unlock(&udc_lock);