iio: adc: ad7280a: handle spi_setup() errors in probe()

iio: adc: ad7280a: handle spi_setup() errors in probe()

The probe() function ignored the return value of spi_setup(), leaving SPI
configuration failures undetected. If spi_setup() fails, the driver should
stop initialization and propagate the error to the caller.

Add proper error handling: check the return value of spi_setup() and return
it on failure.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 2051f25d2a26 ("iio: adc: New driver for AD7280A Lithium Ion Battery Monitoring System")
Signed-off-by: Pavel Zhigulin <Pavel.Zhigulin@kaspersky.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>