iio: gyro: adxrs290: use hook for devm resource unwinding

Make use of devm_add_action_or_reset() hook to switch device into STANDBY
mode during standard resource unwinding. The patch includes a helper
function, in the form of adxrs290_set_mode(), to realise driving the
device into STANDBY mode.

Signed-off-by: Nishant Malpani <nish.malpani25@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200910122711.16670-1-nish.malpani25@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>