staging:iio:cdc:ad7150: Tidy up local variable positioning.

Where there is no other basis on which to order declarations
let us prefer reverse xmas tree.  Also reduce scope where
sensible.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Barry Song <song.bao.hua@hisilicon.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-16-jic23@kernel.org