iio: imu: inv_icm42600: Simplify pm_runtime setup

iio: imu: inv_icm42600: Simplify pm_runtime setup

Rework the power management in inv_icm42600_core_probe() to use
devm_pm_runtime_set_active_enabled(), which simplifies the runtime PM
setup by handling activation and enabling in one step.
Remove the separate inv_icm42600_disable_pm callback, as it's no longer
needed with the devm-managed approach.
Using devm_pm_runtime_enable() also fixes the missing disable of
autosuspend.
Update inv_icm42600_disable_vddio_reg() to only disable the regulator if
the device is not suspended i.e. powered-down, preventing unbalanced
disables.
Also remove redundant error msg on regulator_disable(), the regulator
framework already emits an error message when regulator_disable() fails.

This simplifies the PM setup and avoids manipulating the usage counter
unnecessarily.

Fixes: 31c24c1e93c3 ("iio: imu: inv_icm42600: add core of new inv_icm42600 driver")
Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Link: https://patch.msgid.link/20250901-icm42pmreg-v3-1-ef1336246960@geanix.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>