mhi: core: ensure MHI is active while processing mission mode

It is possible for MHI host to be in suspended state before
processing mission mode and block register access. Ensure MHI is
in active state before processing mission mode transition and
allow boot sequence completion.

CRs-Fixed: 2334309
Change-Id: I62ec8779894451b32f38a7f9129a7121ecfb8bd1
Acked-by: Bhaumik Vasav Bhatt <bbhatt@qti.qualcomm.com>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>