iio: imu: inv_mpu: Move exports to IIO_MPU6050 namespace

As these exports are only relevant to core module and users in the
bus specific modules, move them out of the main kernel namespace.

Includes using EXPORT_NS_GPL_DEV_PM_OPS() and the simplifications that
brings by allowing the compiler to remove unused struct dev_pm_ops
and callbacks without needing explicit __maybe_unused markings.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
Cc: Linus Walleij <linus.walleij@linaro.org>

--
Dropped Linus' tag as the new patch is significantly different.

v2: Switch to more flexible version of EXPORT* macro from Paul.
Acked-by: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
Link: https://lore.kernel.org/r/20220925155719.3316280-6-jic23@kernel.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>