mhi: core: add support to ring doorbell (DB) in any active MHI state

Newer MHI based modems support ringing transfer channel DB
register while in M2 state. Host is not required to ring WAKE DB
to trigger M2 exit. This change takes advantage of it by not
ringing WAKE DB to exit M2 during data transfer.

CRs-Fixed: 2418347
Change-Id: I5deedcada0140b6439d8418124566c6a6b6c7410
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>