xhc-whb04b-6 fixes

[hdiethelm]

Hello

I'm pretty new to LinuxCNC, just finalizing my new CNC mill. I hope this PR matches your expectations. Otherwise, just tell me.

I fixed some issues in the xhc-whb04b-6 component:

• The -B argument was not working. I had 5 and 10 mm steps always active which is a bit dangerous
• I had strange movements after changing from MPG to Step or Continuous mode
• Jogging while a program is running gives unnecessary errors

Somehow, also the Spindle and Feed values are not shown on the display. I was not able to fix this until now. If I find the issue, I will create a separate PR.

[hdiethelm]

I just fixed an other issue: Jogging while a program is running always gave an error. There is no reason to try to change to manual mode when the mode is auto.

[hdiethelm]

I tested in Mach3 in a Windows VM with the original plugin from https://www.cdxhctech.com/html/edownloads/view_420.html and the display also doesn't show any spindle speed or feed. So I think might be a firmware bug of my device. So I leave that for now.

I created a back-port to the latest release if someone (like me) doesn't want to test all new changes on his new CNC: https://github.com/hdiethelm/linuxcnc-fork/tree/xhc-whb04b-6-fix-v2_backport-v2.9.8