serdev: ttyport: do not used keyed wakeup in write_wakeup

Serdev does not use the file abstraction and specifically there will
never be anyone polling a file descriptor for POLLOUT events.

Just use plain wake_up_interruptible() in the write_wakeup callback and
document why it's there.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>