STM32 CAN driver, added assertions and checks.

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/branches/stable_2.6.x@5711 35acf78f-673a-0410-8e92-d51de3d6d3f4
Added missing definitions.

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/branches/stable_2.6.x@5713 35acf78f-673a-0410-8e92-d51de3d6d3f4