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

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

BugLink: https://bugs.launchpad.net/bugs/1837664
[ Upstream commit 0559ef7fde67bc6c83c6eb6329dbd6649528263e ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>