Move ops count to top-level context

When a restartable function calls another restartable function, the current
ops_count needs to be shared to avoid either doing too many operations or
returning IN_PROGRESS uselessly. So it needs to be in the top-level context
rather than a specific sub-context.

include/mbedtls/ecp.h

@@ -184,7 +184,8 @@ typedef struct mbedtls_ecp_restart_mul mbedtls_ecp_restart_mul_ctx;
  */
 typedef struct
 {
-    mbedtls_ecp_restart_mul_ctx *rsm;       /*!<  restart context for ecp_mul()     */
+    unsigned ops_done;                  /*!<  current ops count             */
+    mbedtls_ecp_restart_mul_ctx *rsm;   /*!<  ecp_mul_comb() sub-context    */
 } mbedtls_ecp_restart_ctx;
 #endif /* MBEDTLS_ECP_EARLY_RETURN */