iio:adc:ad7766: Fix unnecessary check in ad7766_probe()

iio:adc:ad7766: Fix unnecessary check in ad7766_probe()

In the function ad7766_probe(), the return value of
devm_iio_device_register() can be zero or ret, Thus it
is unnecessary to repeated check here.

Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Co-developed-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/20210511134739.948-1-tangbin@cmss.chinamobile.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>