staging: comedi: me4000: use comedi_range_is_bipolar() in ai (*insn_read)

Use the helper function to check the range type instead of relying on the
value. For aesthetics, rename the local variable used for the range.

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>

diff --cc drivers/staging/comedi/drivers/me4000.c
Simple merge