port-probe: ensure port is open for all plugin AT probes

If multiple plugins are selected for probing a modem with AT probes,
earlier plugins may only request some probes but not all.  When
those probes complete successfully, subsequent plugins won't re-do
the probes that were already completed, but jump to ones that
haven't yet been done.

For example, plugin A may request only AT probes but plugin B
may request AT, AT_PRODUCT, and AT_VENDOR. Plugin B will start
with AT_PRODUCT probes because plugin A already completed the AT
probe.

The code in probe_step() only opened the serial port for the
AT probe, I guess assuming that all AT-type probes would be
done in sequence. But in the above scenario they are not.

Instead, make sure the port is opened for all AT style probes
that have not yet been executed.

Fixes: 7937a89a port-probe: rework and consolidate port probe flow for AT/QCDM/QMI/MBIM

Signed-off-by: Dan Williams <dan@ioncontrol.co>