ssl_pick_cert: use the public key for can_do

This is in preparation for support of external private key operations,
where there is no private key object.
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 70fe13d..2034d75 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -707,7 +707,7 @@
         MBEDTLS_SSL_DEBUG_CRT( 3, "candidate certificate chain, certificate",
                           cur->cert );
 
-        if( ! mbedtls_pk_can_do( cur->key, pk_alg ) )
+        if( ! mbedtls_pk_can_do( &cur->cert->pk, pk_alg ) )
         {
             MBEDTLS_SSL_DEBUG_MSG( 3, ( "certificate mismatch: key type" ) );
             continue;