mhi: core: for downlink transfer only assert wake if MHI is in lpm mode

When MHI is in an active state, external device CPU cores can go
into a low power mode while device data movers processing data
transfer. Whenever host assert wake, it can cause device CPU to
exit low power mode even though there is no work to be done. For
downlink channels, assert wake only if MHI is in a low power mode
to avoid unnecessary wake ups.

CRs-Fixed: 2248650
Change-Id: I05ca7483a885237711a630844670e887572c13b7
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>