Check for drive using ls

src/freedombone-controlpanel

@@ -1812,7 +1812,7 @@ function performance_benchmarks {
     fi
 
     test_drive=/dev/sda1
-    if [ -f /dev/mmcblk0p2 ]; then
+    if ls /dev/mmcblk0p2; then
         test_drive=/dev/mmcblk0p2
     fi
     echo ''