CAEN/HV: Fix comparision between string and int

Change-Id: I39f7f8e43ac340c6a18cf6c47c288eb92db068cc
Reviewed-on: https://forge.frm2.tum.de/review/c/frm2/tango/entangle/+/26485

Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Jens Krueger <jens.krueger@frm2.tum.de>

entangle/device/caen/hv_ps.py

@@ -210,7 +210,7 @@ class PowerSupply(base.PowerSupply):
         values = ''
         if val:
             value = val[0]
-            if value < 0:
+            if not isinstance(value, str) and value < 0:
                 value = abs(value)
             values = VALUE + str(value)
         reply = self._iodev.Communicate(