mbedtls: fix typo in variable name

Broken a few days ago in 6448f98.

Bug: https://curl.haxx.se/mail/lib-2017-03/0015.html
diff --git a/lib/vtls/mbedtls.c b/lib/vtls/mbedtls.c
index dbcb169..b131713 100644
--- a/lib/vtls/mbedtls.c
+++ b/lib/vtls/mbedtls.c
@@ -159,7 +159,7 @@
 
 static CURLcode mbedtls_version_from_curl(int *mbedver, long version)
 {
-  switch(ssl_version) {
+  switch(version) {
     case CURL_SSLVERSION_TLSv1_0:
       *mbedver = MBEDTLS_SSL_MINOR_VERSION_1;
       return CURLE_OK;