power control: need option to repeat off command

[leandrolanzieri]

It seems that on Linux, some devices are turned back on by the kernel after powering them off. See mvp/uhubctl#96

The proposed workaround is to repeat the command a number of times. A downside is that the number of repetitions seems to depend on the system (e.g., on my work station with the amazon hub I need 500 repetitions). This could be potentially added to the locations description.