staging:iio:accel:adis16240: sign extend function replace hard code duplication

Use sign_extend32 kernel function instead of code duplication,
Safe also for 16 bit. and remove declaration of bits variable not needed.

Signed-off-by: Karim Eshapa <karim.eshapa@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>