tty: n_smux: Remove Sleep Request ACK when powering down

Removed the ACK from the protocol when a sleep request is received when
already in the turning-off state to avoid an extra wakeup just to send
the ACK.

This case happens when both the local and remote sides transition to the
turning-off state and send a sleep request.  Both sides will then
receive the sleep request at which point they can immediately transition
to the off side without having to perform a wakeup just to send the ACK.

Change-Id: I3fe4a82da34b68ef51ee9aa0c00507361b0717ed
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>