Fix: parse_prob_opts return the actual success of the function

This bug have been triggered by the mi merging and the use of a
command_ret in enable_events functions. Previously, enable_events was
reusing the ret variable for another operation and always replacing ret.
Parse_probe_event returned the last output of sscanf which represent
the number of match and not the success of the operation.

Fixes #830

Signed-off-by: Jonathan Rajotte Julien <jonathan.r.julien@gmail.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>