port-probe: fix custom init probing not having open AT port
Referenced commit broke custom init flows because the AT port was not opened when custom inits were run, which was the case before the commit landed.
Rework probing flow to follow a step model like much of our other code, and ensure that the AT port is opened as part of the custom init flow.
Fixes: 7937a89a port-probe: rework and consolidate port probe flow for AT/QCDM/QMI/MBIM
Signed-off-by: Dan Williams dan@ioncontrol.co