staging: comedi: comedidev.h: document remaining inline functions

Add kernel-doc headers to the inline functions that do not have them
yet.

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>