Made optional the STM32 MAC DMABMR SR reset procedure.

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/branches/stable_2.6.x@6868 35acf78f-673a-0410-8e92-d51de3d6d3f4
git-svn-id: svn://svn.code.sf.net/p/chibios/svn/branches/stable_2.6.x@6870 35acf78f-673a-0410-8e92-d51de3d6d3f4