[nrfconnect] Fix flashing script (#15628)

--reset flash should be used to reset the board after
flashing a firmware.

scripts/flashing/nrfconnect_firmware_utils.py

@@ -130,7 +130,7 @@ def flash(self, image):
 
     def reset(self):
         """Reset the device."""
-        return self.run_tool('nrfjprog', ['--pinresetenable'], name='Enable pin reset')
+        return self.run_tool('nrfjprog', ['--reset'], name='Reset')
 
     def actions(self):
         """Perform actions on the device according to self.option."""