staging: comedi: comedidev.h: tweak docs for inline functions

Tweak the kernel-doc for already documented inline functions for
consistency, and document their return values.

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>