tty: n_smux: fix deadlock between RX and TX workers

smux_handle_rx_open_ack function acquires the channel state spin lock
and tries to add the channel into ready list causing spin lock lockup
between smux_rx_worker thread and smux_tx_worker thread running on
two different CPUs.

Add the channel to ready list after releasing the state spin lock.

CRs-Fixed: 514726
Change-Id: I11729290c5c28a13bed66cf7506b8aaf48a32d2a
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>