greybus: svc: add power mode call for link hibernation

Due to when using set_power_mode to hibernate a link, it won't trigger a
POWERMODEIND event, hence the hard-coded GB_SVC_SETPWRM_PWR_OK would be
returned and it should also be considered as successful result code for
link hibernation. Therefore, adding this set_power_mode_hibernate
function to separate the two calls in order to check with the correct
result code.

Testing Done:
 - Suspend an Interface and observe no set power mode error.

Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Alex Elder <elder@linaro.org>