mhi: core: check MHI host state instead of MHI dev state for LPM exit state

When a client request to disable MHI low power mode, it's
important MHI host only return after exiting MHI low power modes.
Otherwise, clients may get un-clocked register access when
accessing physical link. By checking host state, we're guaranteed
to return only after exiting low power states.

CRs-Fixed: 2320851
Change-Id: I45f5f8463fba2c35ccd6c9e67e749d9d4fc3d904
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>