greybus: arche-platform: Avoid doing same thing again in poweroff fn

If user switches from fw_flashing => off mode, then we do not need
to do same things again, for example, clk_disable and wake/detect event,
as while switching to fw_flashing, driver makes sure that device goes
to off state and then brings back in fw_flashing state.

Testing Done: Tested on DB3.5 platform.

Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>