Reset ops_done at the right time

This should only be done in the top-level function.

Also, we need to know if we indeed are the top-level function or not: for
example, when mbedtls_ecp_muladd() calls mbedtls_ecp_mul(), the later should
not reset ops_done. This is handled by the "depth" parameter in the restart
context.

include/mbedtls/ecp.h

@@ -185,6 +185,7 @@ typedef struct mbedtls_ecp_restart_mul mbedtls_ecp_restart_mul_ctx;
 typedef struct
 {
     unsigned ops_done;                  /*!<  current ops count             */
+    unsigned depth;                     /*!<  call depth (0 = top-level)    */
     mbedtls_ecp_restart_mul_ctx *rsm;   /*!<  ecp_mul_comb() sub-context    */
 } mbedtls_ecp_restart_ctx;
 #endif /* MBEDTLS_ECP_EARLY_RETURN */