usb: remove unnecessary NULL device check from usb_ep_get()

No caller of usb_ep_get() calls it with a NULL device (previous commits
have addressed the few remaining cases which didn't explicitly check).
Replace check for 'dev == NULL' with an assert instead.

Signed-off-by: Liam Merwick <liam.merwick@oracle.com>
Message-id: 1549460216-25808-10-git-send-email-liam.merwick@oracle.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb/core.c

@@ -717,9 +717,7 @@ struct USBEndpoint *usb_ep_get(USBDevice *dev, int pid, int ep)
 {
     struct USBEndpoint *eps;
 
-    if (dev == NULL) {
-        return NULL;
-    }
+    assert(dev != NULL);
     if (ep == 0) {
         return &dev->ep_ctl;
     }