mailbox protocol: handle potential errors in the ack phase better
mailbox protocol: handle potential errors in the ack phase better

One of the error return-codes for ack_mailbox() was being discarded.  Fix that.

Testing Done: Bring up firmware on apb2
Bug Number: Trivial
Review URL: https://reviews.leaflabs.com/r/1560/
Reviewed by: Morgan Girling <girling_morgan@projectara.com>

Signed-off-by: Eli Sennesh <5ebe47bba478951130dff4ea23416219c2dfa45d@leaflabs.com>

projectara/bootrom,gregkh/bootrom