lchan_rtp_fsm: fix out_state_mask

In the state LCHAN_RTP_ST_WAIT_MGW_ENDPOINT_CONFIGURED, the event
LCHAN_RTP_EV_ROLLBACK is allowed and also handled in the action
callback, which causes a change to LCHAN_RTP_ST_ROLLBACK. However,
LCHAN_RTP_ST_ROLLBACK is missing from the out_state_mask.

Change-Id: Ifca3892901c8389beee6e4f0fea03c33cfbdc265
1 file changed