firmware_Cr50RMAOpen: fix obsolete using_servo_v4

During a recent refactor, this method was left dangling. The original
method inside chrome_cr50.py was replace with a call to
dts_mode_is_valid(). This change adopts the same logic here.

BUG=b:149754300
TEST=test_that --autotest_dir . $DIP firmware_Cr50RMAOpen --args
     "servo_host=$LIP"

/tmp/test_that_results_fHx6tm/results-1-firmware_Cr50RMAOpen
[  PASSED  ]

Change-Id: I4ab074390c919d04a62ccc0cbea13257d0185b1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/2067343
Tested-by: Ruben Rodriguez Buchillon <coconutruben@chromium.org>
Reviewed-by: Mary Ruthven <mruthven@chromium.org>
Commit-Queue: Ruben Rodriguez Buchillon <coconutruben@chromium.org>
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.')