staging: comedi: COMEDI_CB_EOA is also used to report end-of-output.

Update comments in comedi.h accordingly.

Signed-off-by: W. Trevor King <wking@drexel.edu>
Acked-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>