iio:magn:ak8975: Drop of_match_ptr and ACPI_PTR protections.

Both would result in only a small size saving.  For simplicity it
is best to remove them.  I also wish to remove both these antipatterns
from IIO.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Tested-by: Jonathan Albrieux <jonathan.albrieux@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Jonathan Albrieux <jonathan.albrieux@gmail.com>
Link: https://lore.kernel.org/r/20200910173242.621168-35-jic23@kernel.org