mhi: core: marked __packed only members that's required

It's recommended style to mark __packed only members that
does not have natural alignment for clarity.

CRs-Fixed: 2258358
Change-Id: Id560e23427d932c9e15cc7e4872366f130d7e8df
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>