fix sequencing in secop_demo.modules.MagnetiField

Change-Id: Idfd8746782b2bba61bfb30cba1010000f41aed75
Reviewed-on: https://forge.frm2.tum.de/review/19201
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>

secop_demo/modules.py

@@ -126,6 +126,7 @@ class MagneticField(Drivable):
         return self.value
 
     def write_target(self, value):
+        self.status = self.Status.BUSY, 'setting target'
         # could tell HW
         return round(value, 2)
         # note: setting self.target to the new value is done after this....