staging: comedi: comedidev.h: use enum for COMEDI_CB_... constants

The existing constants `COMEDI_CB_EOS` etc. are in the form of macros
and have a bogus kernel-doc comment.  Change them to `enum` constants so
they can be documented properly with kernel-doc.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>