mesh: Rename MPM FSM states to match the standard

mesh: Rename MPM FSM states to match the standard

During the P802.11s draft development, there were separate LISTEN and
IDLE states. However, the current IEEE 802.11 standards uses only the
IDLE state while the implementation called this LISTEN. Rename the state
in the implementation to match the one used in the standard to avoid
confusion. In addition, rename OPEN_{SENT,RCVD} to OPN_{SNT,RCVD} to
match the exact spelling of these states in the standard.

Signed-off-by: Jouni Malinen <j@w1.fi>