iio:magn:ak8974: Drop of_match_ptr protection

This prevents use of this driver with ACPI via PRP0001 and is
an example of an anti pattern I'm trying to remove from IIO.
Hence drop from this driver.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200910173242.621168-36-jic23@kernel.org