ssl_server2: Skip CA setup if `ca_path` or `ca_file` argument "none"

This allows to test PSK-based ciphersuites via ssl_server2 in builds
which have MBEDTLS_X509_CRT_PARSE_C enabled but both MBEDTLS_FS_IO and
MBEDTLS_CERTS_C disabled.
diff --git a/programs/ssl/ssl_server2.c b/programs/ssl/ssl_server2.c
index d370892..f5d370b 100644
--- a/programs/ssl/ssl_server2.c
+++ b/programs/ssl/ssl_server2.c
@@ -2022,17 +2022,17 @@
     mbedtls_printf( "  . Loading the CA root certificate ..." );
     fflush( stdout );
 
+    if( strcmp( opt.ca_path, "none" ) == 0 ||
+        strcmp( opt.ca_file, "none" ) == 0 )
+    {
+        ret = 0;
+    }
+    else
 #if defined(MBEDTLS_FS_IO)
     if( strlen( opt.ca_path ) )
-        if( strcmp( opt.ca_path, "none" ) == 0 )
-            ret = 0;
-        else
-            ret = mbedtls_x509_crt_parse_path( &cacert, opt.ca_path );
+        ret = mbedtls_x509_crt_parse_path( &cacert, opt.ca_path );
     else if( strlen( opt.ca_file ) )
-        if( strcmp( opt.ca_file, "none" ) == 0 )
-            ret = 0;
-        else
-            ret = mbedtls_x509_crt_parse_file( &cacert, opt.ca_file );
+        ret = mbedtls_x509_crt_parse_file( &cacert, opt.ca_file );
     else
 #endif
 #if defined(MBEDTLS_CERTS_C)