mhi: cntrl: qcom: read power down state from boot monitor

Boot monitor thread needs to wake up when core layer starts a
power down. If the execution environment value gets overwritten
by the time the thread wakes up, it will keep waiting until it
times out. Read the power down state instead to reliably exit
whenever core layer issues the wake up.

Change-Id: I9b629ce1f4f720fb1e28fe1b371666dc57264a28
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>