tty: n_smux: Fix packet close synchronization issue

If a channel is logical channel open or close ack is pending in the
transmit queue when a logical channel is closed, then they are currently
purged and the remote and local channel states are mismatched and the
port cannot be re-opened.  Note that this issue only occurs during rapid
open/close/open sequences that are typically only seen in remote
loopback stress testing.

Add code to not purge ACK commands from a logical channel unless the
logical channel purge is due to a subsystem restart (at which point the
remote state is known to be power-on default).

Change-Id: I52763323642bb7c505630bb994ecc1e021270d17
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>