iio: proximity: srf04: Use pm_ptr() to remove unused struct dev_pm_ops

iio: proximity: srf04: Use pm_ptr() to remove unused struct dev_pm_ops

If CONFIG_PM is not set, the pm_ptr() will ensure that the struct
dev_pm_ops and callbacks are removed without the need for __maybe_unused
markings.

In this case we can't simply use DEFINE_RUNTIME_DEV_PM_OPS() because
that would provide suspend and resume functions without the
checks the driver is doing before calling runtime_pm functions
(whether the necessary GPIO is provided).  It may be possible to
clean that up in future by moving the checks into the callbacks.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Andreas Klinger <ak@it-klinger.de>
Link: https://lore.kernel.org/r/20220807185618.1038812-5-jic23@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>