Fixed bug #492.

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/branches/kernel_3_dev@6897 35acf78f-673a-0410-8e92-d51de3d6d3f4
git-svn-id: svn://svn.code.sf.net/p/chibios/svn/branches/kernel_3_dev@6900 35acf78f-673a-0410-8e92-d51de3d6d3f4