staging: comedi: ni_labpc: simplify labpc_range_is_unipolar()

As Ian Abbott pointed out, this helper function can be simplified
by just checking if the 'min' value is >= 0.

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>