staging:iio:imu fix missing register table index for some channels

A number of these register indexes were not set or actually wrong.
This only effects sysfs reads.

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