tty: n_smux: Allow TX work to run while waiting for wakeup

The wakeup worker and TX worker both run on the same single-threaded
workqueue, so the wakeup worker needs to reschedule after sending a
wakeup character to allow pending characters to get flushed.

Without this change, a burst of wakeup characters will be sent once the
exponential back-off delay is 32ms after which the wakeup worker
reschedules itself as a delayed work item.

Change-Id: I4260c7f5b2f1f90e87575eac4b407be2bc6adb1d
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>