staging: comedi: rti800: tidy up CR_{CHAN,RANGE} usage

The insn->chanspec passed to the CR_{CHAN,RANGE} macros is an unsigned
int. For aesthetic reasons, put the result of the macros into an
unsigned int local var.

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