[En-Nut-Discussion] USART RINGBUF Structure Change

[Harald Kipp]

Some members of the RINGBUF structure are modified
in interrupt context but were not marked volatile.

I changed this in CVS HEAD. No idea if this may have
caused any problems in the past, but the driver passed
a long time test with this modification here.

Harald

P.S. Matthias, I'm sorry for having spoiled the
old-style CTS definition in a previous change.