staging: iio: adc: ad7280a: handle error from __ad7280_read32()

Inside __ad7280_read32(), the spi_sync_transfer() can fail with negative
error code. This change will ensure that this error is being passed up
in the call stack, so it can be handled.

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