Low frequency support

[ahhuhtal]

NanoVNA v2 is capable of measuring down to 10kHz after the 20200525 update. This works fine on the local UI.

However, the frequency sweep menu allows setting the start and stop frequencies only down to 0.1 MHz. The serial protocol takes input in Hertz, so commanding the low frequencies should not be a problem.

Thus this seems to only be an issue of input sanitization in the sweep menu.