configure: Fix checking of six library for Python 3.

Copied from python 2 checker but not corrected.

Fixes: 8fb7d02 ("configure: Check for presence of Python 3.")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Russell Bryant <russell@ovn.org>

m4/openvswitch.m4

@@ -382,9 +382,9 @@ else:
             fi
           done
         done
-        if test $ovs_cv_python != no; then
-          if test -x "$ovs_cv_python" && ! "$ovs_cv_python" -c 'import six' >/dev/null 2>&1; then
-            ovs_cv_python=no
+        if test $ovs_cv_python3 != no; then
+          if test -x "$ovs_cv_python3" && ! "$ovs_cv_python3" -c 'import six' >/dev/null 2>&1; then
+            ovs_cv_python3=no
             AC_MSG_WARN([Missing Python six library.])
           fi
         fi