fix getting current idle driver error (#9799)

Signed-off-by: Xichen Lin <[email protected]>

tests/platform_tests/test_idle_driver.py

@@ -17,8 +17,8 @@
 
 def test_idle_driver(duthosts, enum_rand_one_per_hwsku_hostname):
     duthost = duthosts[enum_rand_one_per_hwsku_hostname]
-    idle_driver = duthost.shell('cat /sys/devices/system/cpu/cpuidle/current_driver')['stdout']
-    if idle_driver != "none":
+    idle_driver_result = duthost.shell('cat /sys/devices/system/cpu/cpuidle/current_driver', module_ignore_errors=True)
+    if idle_driver_result['rc'] == 0 and idle_driver_result['stdout'] != "none":
         cstates = duthost.shell('sed -n "s/.*C\\([0-9]*\\).*/\\1/p" '
                                 '/sys/devices/system/cpu/cpu*/cpuidle/state*/name')['stdout'].split()
         max_cstate = max([int(cstate) for cstate in cstates])