lacplib: set the max priority to LACP flow entries

Changes v1 -> v2:

- correct the following expression

The measure against the default priority of OFPFlowMod of OFP1.2/1.3 being 0.

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>