tty: n_smux: Add Dedicated Power Control Queue

The current design uses the existing channel queues for sending power
commands along with a command filter in the TX worker that always sends
all command messages for queues as long as the TTY UART link is up.
This can allow open and close events to be sent between a request to
power down the link and before the ACK has been received that actually
powers down the link.

Update the implementation to add a dedicated control channel for
power-control commands and bytes.  All power commands and bytes are now
sent by the TX worker that properly prioritizes the power commands over
logical channel commands.

CRs-Fixed: 369044
Change-Id: I05c080885c79c3510d02cc360bdfe879d68a962c
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>