net: ipa: rename "tag status" symbols

There is a set of functions and symbols related to performing
"tag_process" immediate commands to clear the IPA pipeline.  The
name is related to one of the commands issued when doing this, but
it doesn't really convey the overall purpose of taking this action.

The purpose is to take some steps to "clear out" the hardware
pipeline, and to wait until that process completes, to ensure the
IPA hardware is in a well-defined state.

Rename these symbols to use "pipeline_clear" in their names instead.
Add some comments to explain a bit more about what's going on.

Signed-off-by: Alex Elder <elder@linaro.org>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>