staging: comedi: comedidev.h: introduce some 'range_is_external' helpers

The comedi_krange includes a flags member that currently identifies the
'units' of the range (RF_UNIT) and if the range is from an internal or
external source (RF_EXTERNAL).

Introduce some helper functions to check if a given range is from an
external source.

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