iio:magn:ak8975: Switch from CONFIG_PM guards to pm_ptr() etc

Letting the compiler remove these functions when the kernel is built
without CONFIG_PM support is simpler and less error prone than the
use of #ifdef based config guards.

Use the new DEFINE_RUNTIME_DEV_PM_OPS() macro to reduce boilerplate.

Removing instances of this approach from IIO also stops them being
copied into new drivers.

Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Jonathan Albrieux <jonathan.albrieux@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220130193147.279148-30-jic23@kernel.org