usbnet: ipheth: fix potential null pointer dereference in ipheth_carrier_set

_dev_ is being dereferenced before it is null checked, hence there
is a potential null pointer dereference.

Fix this by moving the pointer dereference after _dev_ has been null
checked.

Addresses-Coverity-ID: 1462020
Fixes: bb1b40c7 ("usbnet: ipheth: prevent TX queue timeouts when device not ready")
Signed-off-by: NGustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/usb/ipheth.c

@@ -291,12 +291,15 @@ static void ipheth_sndbulk_callback(struct urb *urb)
 
 static int ipheth_carrier_set(struct ipheth_device *dev)
 {
-	struct usb_device *udev = dev->udev;
+	struct usb_device *udev;
 	int retval;
+
 	if (!dev)
 		return 0;
 	if (!dev->confirmed_pairing)
 		return 0;
+
+	udev = dev->udev;
 	retval = usb_control_msg(udev,
 			usb_rcvctrlpipe(udev, IPHETH_CTRL_ENDP),
 			IPHETH_CMD_CARRIER_CHECK, /* request */