dp8393x: Flush packets when link comes up

.can_receive callback changes semantics that once return 0, backend will
try sending again until explicitly flushed, change the device to meet
that.

dp8393x_can_receive checks SONIC_CR_RXEN bit in SONIC_CR register and
SONIC_ISR_RBE bit in SONIC_ISR register, try flushing the queue when
either bit is being updated.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Message-id: 1436955553-22791-12-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
