staging:iio:adxrs450: Perform sign extension for the calibbias register

The calibbias (DNC) register contains a 10-bit twos complement value. Perform a
proper sign extension when reading the register, otherwise negative will
incorrectly be displayed as large positive values.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>