greybus: operation: fix operation-destroy race

Make sure to acquire the connection-list lock atomically when releasing
the final reference.

This allows the list to be traversed and references to be acquired
(while holding the lock) without racing with the destructor.

Suggested-by: Greg Kroah-Hartman <gregkh@google.com>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>