[LinuxCNC/linuxcnc PR#520] Issue159 2.7

Fix issue #159 for 2.7. This is the same as #184, just cherry picked. Noted by @andypugh in #500.j

Testing performed: Make selections in pncconf until it is possible to arrive at the “X Motor” screen with the “Test / Tune Axis” button. Click it.

Before the change, the reported error occurs.

After the change, an error occurs when trying to load hm2_pci, which is expected since no such device is attached.