mhi: core: Remove race condition on LPM

Eliminate race condition whereby the device could go into
a low power mode before the host has finished initializing.

CRs-Fixed: 836441
Change-Id: I9957e87b1bfe956963c1f74f0a02d6714e9afcbe
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>