net: pcs: xpcs-plat: fix runtime PM initialization

The driver calls pm_runtime_set_active() before runtime PM is enabled and
before the CSR clock is prepared and enabled. If the device is later
suspended (e.g. during unbind), the runtime suspend callback tries to
disable a clock that was never enabled, producing:

    clk_csr already disabled
    clk_csr already unprepared

Set the initial runtime PM status to suspended instead, and register a
devm action that force-suspends the device on teardown. Without that
action the CSR clock would stay enabled if the driver is removed while
runtime-active, since nothing triggers a runtime suspend on unbind.
pm_runtime_force_suspend() is not checked because the runtime suspend
callback cannot fail.

The clock is enabled on the first runtime resume (e.g. during MDIO
access).

No upstream platform currently enables this driver with a real CSR clock
dependency, so the issue is not reachable in mainline today. It was
found while using this driver as a reference for the Rockchip XPCS glue
driver.

Signed-off-by: Coia Prant <coiaprant@gmail.com>
Link: https://patch.msgid.link/20261023075214.1793922-3-coiaprant@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>