greybus: es2: move APB log task into the es2_ap_dev struct

If an APB has a logging task it is associated with that APB.  Move
the task pointer into the es2_ap_dev structure rather than having it
be a single global.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>