tty: n_smux: After receiving OPEN_ACK flush tx_queue

If a local SMUX client re-opens the channel and tries to send data before
receiving the OPEN_ACK, the transaction will get stuck because the
channel local state is not OPENED.

Add the channel to ready list if tx_queue is not empty after receiving
the OPEN_ACK.

CRs-Fixed: 507379
Change-Id: I357b6edc340c404c890da0c415ed3aeba96dcfe8
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>