[smio:acq] Triggered acquisiton does not support post_samples = 0

[lerwys]

Problem: Currently, the gateware does not support triggered acquisition (i.e., external, data or sw) with post_samples = 0. This is probably related to the fact that we align the trigger signal with the first post_trigger valid sample.

This is related to lnls-dig/bpm-gw#62 and will be mitigated by limiting in software the minimum number of post-samples