Add return to iter_to_dataframe when filtering channels

[hugabora]

Bugfix:

mdf iter_to_dataframe takes channels parameter to allow filtering. In this case a filtering is set for the mdf and same member function is invoked. However at the end of this if clause there is no return, effectively continuing into the "else" codepath instead of stopping the iteration. Effectively we get duplicated / more rows.

Added return.

fyi: @danielhrisca

[danielhrisca]

Thank you !

[hugabora]

Thank you as well!