staging: comedi: cb_das16_cs: use the BIP_RANGE helper macro

The BIP_RANGE(a) macro can be used instead of the RANGE(a,b)
macro when -a == b. And it's a bit clearer that this is a bipolar
range.

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