Fix POP_MPLS of ofctl_v1_3.to_actions() problem

POP_MPLS (class OFPActionPopMpls) requires ethertype.

Signed-off-by: Satoshi Kobayashi <satoshi-k@stratosphere.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>