staging: comedi: comedi.h: Fix comment for struct comedi_krange

The kernel-doc comment for `struct comedi_krange` refers to the macro
constant `RF_external`.  It should be `RF_EXTERNAL`, so fix it.  It also
documents the value of the constant as `(1 << 8)`, but the macro now
expands to the hexadecimal constant `0x100`, so use that as the
documented value.

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>