iio: at91-sama5d2: Fix incorrect cast to platform_device

iio: at91-sama5d2: Fix incorrect cast to platform_device

The at91-sama5d2 driver calls `to_platform_device()` on a struct device
that is part of a IIO device. This is incorrect since
`to_platform_device()` must only be called on a struct device that is part
of a platform device.

The code still works by accident because non of the struct platform_device
specific fields are accessed.

Refactor the code a bit so that it behaves identically, but does not use
the incorrect cast. This avoids accidentally adding undefined behavior in
the future by assuming the `struct platform_device` is actually valid.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lore.kernel.org/r/20211019082929.30503-1-lars@metafoo.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>