mhi: core: Move power down check within the RDDM image support

If a controller supports RDDM, there is no need to process a SYS_ERROR.
Hence, we can safely move the power down state check to reside within an
RDDM supported controller as it allows for other controllers to handle
scenarios such as receiving an early SYS_ERROR when in the PBL execution
environment.

Change-Id: I5c63a861b9dd178f5c967bdf252da9c47d3a2b56
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>