mhi_dev: mhi: Update initialization sequence

As part of receiving a callback once the IPA driver is
ready, move the MHI initialization sequence of setting
device ready bit after IPA HW is ready. Once the callback
is received the IPA DMA is used for caching control and data
from the MHI host.

Change-Id: I0ec3b64f3407395bf749a63b521feb84ed46d9f9
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>