IIO: GYRO: ADXRS450: Fix sign issues, properly shift results and limit values

RATE and QUADRATURE_CORRECTION data is formatted as a twos complement number,
and therefore must be handled as type signed short.

TEMP result should be properly shifted.

Dynamic Null Correction is a 10-bit signed number.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>