f_phonet: fix page offset of first received fragment

We pull one byte (the MAC header) from the first fragment before the
fragment is actually appended. So the socket buffer length is 1, not 0.
Signed-off-by: NRémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/usb/gadget/f_phonet.c

@@ -346,7 +346,7 @@ static void pn_rx_complete(struct usb_ep *ep, struct usb_request *req)
 		}
 
 		skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, page,
-				skb->len == 0, req->actual);
+				skb->len <= 1, req->actual);
 		page = NULL;
 
 		if (req->actual < req->length) { /* Last fragment */