ipheth: potential null dereferences on error path

The calls to usb_free_buffer() dereference rx_urb and tx_urb in the
parameter list but those could be NULL.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: L. Alberto Giménez <agimenez@sysvalve.es>
Signed-off-by: David S. Miller <davem@davemloft.net>