update flow cookie

flow cookie layout (mainly used for debugging)
0x0:                         drop invalid packet
0xffffffffffffffff:          unexpected packet received
<port-id(width 32bit)><00(width 8bit)><slice-id(width 32bit)>:
                             normal
                             ex.
                             slice-id := 123, port-id :=1 => 0x10000007b
<port-id(width 32bit)><80(width 8bit)><slice-id(width 32bit)>:
                             flow of mac learning
                             ex.
                             slice-id := 123, port-id :=1 => 0x18000007b