firmware_Cr50RMAOpen: fix obsolete using_servo_v4 am: 92b393374b

Change-Id: Ic6f67337770c584cae3e7a546277fea1f698165b
diff --git a/server/site_tests/firmware_Cr50RMAOpen/firmware_Cr50RMAOpen.py b/server/site_tests/firmware_Cr50RMAOpen/firmware_Cr50RMAOpen.py
index 1c24c80..11d20fa 100644
--- a/server/site_tests/firmware_Cr50RMAOpen/firmware_Cr50RMAOpen.py
+++ b/server/site_tests/firmware_Cr50RMAOpen/firmware_Cr50RMAOpen.py
@@ -65,7 +65,7 @@
         if not self.cr50.has_command('rma_auth'):
             raise error.TestNAError('Cannot test on Cr50 without RMA support')
 
-        if not self.cr50.using_servo_v4():
+        if not self.cr50._servo.dts_mode_is_valid():
             raise error.TestNAError('This messes with ccd settings. Use flex '
                     'cable to run the test.')