PM / Domains: Return early for all errors in _genpd_power_off()

It is strange to only return early for -EBUSY state and left other
errors to be still measured execution time.

As for error cases, the elapsed_ns computed actually is not quite
accurate and meaningful for governor to use. So let's simply return
for all error cases.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>