mhi: core: add a separate callback handler for downlink DTR channel

As part of boot up sequence, some external modems transfer
initial device settings to host due to legacy requirements.
Since we cannot prevent this, silently accept the packets
and discard it.

CRs-Fixed: 2238249
Change-Id: I5d8be63fe54b86fa9d5cbf9a70288eab9f6f2fe4
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>