usb: gadget: u_ether: fix another dereference after null check

dev->port_usb is checked for null pointer previously, so dev->port_usb
might be null during no zlp check, fix it by adding null pointer check.
Acked-by: Jim Baxter <jim_baxter@mentor.com>
Signed-off-by: Harish Jenny K N <harish_kandiga@mentor.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/u_ether.c

@@ -571,7 +571,8 @@ static netdev_tx_t eth_start_xmit(struct sk_buff *skb,
 	req->complete = tx_complete;
 
 	/* NCM requires no zlp if transfer is dwNtbInMaxSize */
-	if (dev->port_usb->is_fixed &&
+	if (dev->port_usb &&
+	    dev->port_usb->is_fixed &&
 	    length == dev->port_usb->fixed_in_len &&
 	    (length % in->maxpacket) == 0)
 		req->zero = 0;