scpi-pps: Data capture dynamically-probed channels

scpi-pps: Data capture dynamically-probed channels

For the few device profiles that probe the device dynamically to determine
channel settings, the results end up in the dynamically-allocated
devc->channels instead of in the statically-configured
devc->device->channels field.

Previously this would crash by dereferencing a null ch_spec for the
device types that do dynamic channel configuration.

[ gsi: address minor style nits ]