vt6420 has the same FIFO overflow problem as vt6421 when combined with
certain devices.  This patch applies the magic fix to vt6420 too.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Martin Qvist <q@maq.dk>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: stable@kernel.org
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>