msm: mhi_dev: Hold a wakelock during data transfer

Modem host interface (MHI) device supports software
channel data transfers. Hold a wakelock if MHI device
is in active state and host has not placed the device
in suspend.

Change-Id: I5000e8b9c6761c6f662ee01d557b78bc16fe1632
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>