Raising exception for get_target_speed() for PSU-fan in PDDF (#8129)

Author: FuzailBrcm

platform/pddf/platform-api-pddf-base/sonic_platform_pddf_base/pddf_fan.py

@@ -224,7 +224,7 @@ def get_target_speed(self):
         target_speed = 0
         if self.is_psu_fan:
             # Target speed not usually supported for PSU fans
-            target_speed = 0
+            raise NotImplementedError
         else:
             idx = (self.fantray_index-1)*self.platform['num_fans_pertray'] + self.fan_index
             attr = "fan" + str(idx) + "_pwm"