hameg-hmo: Release enabled channels when acquisition start fails

Commit db81fbb58261 made sure to release a potentially previously
allocated list of enabled channels before (re-)building the list in the
current invocation of acquisition start.

This commit frees the memory in the error path near the failed creation
already, which reduces the period of time where unused resources are
held, and eliminates a memory leak when acquisition is not stopped after
failed start.

Both approaches can coexist. Freeing an empty list is perfectly fine.