greybus: arche-apb-ctrl: Set default state value to OFF

Explicitly set default apb->state value to ARCHE_PLATFORM_STATE_OFF
in probe() fn.

Testing Done: Tested on EVT1.2 and DB3.5 platform

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