power_domain: Update return value of set_state_async

The function set_state_async always defers requests, so the
correct status to be returned is FWK_PENDING instead of
FWK_SUCCESS.
This change fixes this.

Change-Id: Id27e7b0a65712114e61ff23f16f18fa9219858df
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>