fix set_frequency_sweep command
fix set_frequency_sweep command

The set_sweep_n_points command set the number of points as the channel.
Fixed by giving the channel as the first argument, as for the other SCPI
commands.

jhillairet/scikit-rf,jhillairet/scikit-rf,temmeand/scikit-rf,scikit-rf/scikit-rf,scikit-rf/scikit-rf,temmeand/scikit-rf,temmeand/scikit-rf,jhillairet/scikit-rf