staging: iio: adc: ad7280a: power down the device on error in probe

Power down the device if anything goes wrong after the SPI has been
setup correctly in the probe function.

Existing code that toggles the AD7280A_CTRL_LB_SWRST bit inside
ad7280_chain_setup function is responsible for powering up the device.

Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>