bus: sunxi-rsb: Fix shutdown

Function sunxi_rsb_hw_exit() is sometimes called with pm runtime
disabled, so in such cases pm_runtime_resume() will fail with -EACCES.

Instead of doing whole dance of enabling pm runtime and thus clock just
to disable it again immediately, just check if disabling clock is
needed. That way calling pm_runtime_resume() is not needed at all.

Fixes: 4a0dbc12 ("bus: sunxi-rsb: Implement runtime power management")
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20211121083537.612473-1-jernej.skrabec@gmail.com