Fix overflow in mbedtls_timing_hardclock

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

programs/test/benchmark.c

@@ -398,7 +398,7 @@ static unsigned long mbedtls_timing_hardclock( void )
     }
 
     gettimeofday( &tv_cur, NULL );
-    return( ( tv_cur.tv_sec  - tv_init.tv_sec  ) * 1000000
+    return( ( tv_cur.tv_sec  - tv_init.tv_sec  ) * 1000000U
           + ( tv_cur.tv_usec - tv_init.tv_usec ) );
 }
 #endif /* !HAVE_HARDCLOCK */