driver/serialdigitaloutput: fix get() for DTR signal

If self.signal is "dtr", a ValueError is raised.  That happens because
the if/else logic is wrong for that case. Fix that.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

labgrid/driver/serialdigitaloutput.py

@@ -41,7 +41,7 @@ def __attrs_post_init__(self):
     def get(self):
         if self.signal == "dtr":
             val =  self._p.dtr
-        if self.signal == "rts":
+        elif self.signal == "rts":
             val = self._p.rts
         else:
             raise ValueError("Expected signal to be dtr or rts")