usb: gadget: Fix return value check in ep_write()

In case of error, the function memdup_user() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check
should be replaced with IS_ERR().
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/legacy/inode.c

@@ -440,7 +440,7 @@ ep_write (struct file *fd, const char __user *buf, size_t len, loff_t *ptr)
 
 	value = -ENOMEM;
 	kbuf = memdup_user(buf, len);
-	if (!kbuf) {
+	if (IS_ERR(kbuf)) {
 		value = PTR_ERR(kbuf);
 		goto free1;
 	}