sunvnet: fix potential NULL pointer dereference

One of the error cases for vnet_start_xmit()'s "out_dropped" label
is port == NULL, so only mess with port->clean_timer when port is not NULL.

Signed-off-by: David L Stevens <david.stevens@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>