staging: comedi: comedidev.h: tweak kernel-doc for struct comedi_async

Tweak the formatting of the kernel-doc formatted comment for `struct
comedi_async` in order to stop it generating bogus headings for text
before a colon.

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>