mhi: controller: always set link to D3Hot when powering off

Link must be in D3HOT prior to transitioning to D3Cold state,
otherwise endpoint device may transition into an error state.

CRs-Fixed: 2302001
Change-Id: I97da3be021bdf3232a1f791fb9813fec9e9d8ebd
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>