iio: adc: ad7291: Fix indentation error by adding extra spaces

Added extra spaces before statements to fix
following indentation warnings reported by checkpatch.pl.

WARNING: Statements should start on a tabstop
+           return 0;
Signed-off-by: NAnup Sharma <anupnewsmail@gmail.com>
Link: https://lore.kernel.org/r/Y9Vf4Tp8JKvy+y0u@yogaSigned-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>