mhi: core: fix null pointer dereference during channel reset

During MHI device destroy, MHI host does a channel reset.
As part of channel reset, MHI device drivers requires
access to mhi device pointer associated with current channel.
Defer setting mhi device to null until channel reset is complete.

CRs-Fixed: 2237717
Change-Id: Ic3fb99e17c01c305fb521cc9fe59e7a1da7cc592
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>