Bluetooth: Remove unnecessary smp_mb__{before,after}_atomic

The barriers are unneeded; wait_woken() and woken_wake_function()
already provide us with the required synchronization: remove them
and document that we're relying on the (implicit) synchronization
provided by wait_woken() and woken_wake_function().

Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Reviewed-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>