ssl-opt.sh: Fix getting the list of supported ciphersuites.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 401b89d..55e465a 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -279,7 +279,7 @@
 
 # Space-separated list of ciphersuites supported by this build of
 # Mbed TLS.
-P_CIPHERSUITES=" $($P_CLI --help 2>/dev/null |
+P_CIPHERSUITES=" $($P_CLI help_ciphersuites 2>/dev/null |
                    grep TLS- |
                    tr -s ' \n' ' ')"
 requires_ciphersuite_enabled() {