mgcp: Fix uplink activation of Osmux stream
Commit 575420637981828b64c1292ada015d7170b89390 introduced
OSMUX_STATE_NEGOTIATING to fix a race condition present in osmo-bsc_nat.
However, after this change osmo-bsc_mgcp cannot switch to
OSMUX_STATE_ACTIVATING anymore, which means during osmux_send_dummy time
it won't call osmux_enable_endpoint(), which in turn won't set endp type
to MGCP_OSMUX_BSC.
If MGCP_OSMUX_BSC is not set, uplink streams are sent using regular RTP
instead of Osmux not matter it is enabled in config or not.
Change-Id: Ibcb59aa1ca25408f82cc88c2d5b81177b5f276dc