Fixed typos in USARTv1 driver.

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@5916 35acf78f-673a-0410-8e92-d51de3d6d3f4
Added note.

git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@5917 35acf78f-673a-0410-8e92-d51de3d6d3f4