mhi: core: Enable both time synchronization methods to co-exist

Due to co-existence of both time synchronization methods on a
platform, it is likely that a doorbell method response can be pending
while another host client requests for time using the MMIO or
synchronous method. Wait for completion of the doorbell method and
return the local and remote times from that request as the device may
not be able to handle both requests or host is likely to end up reading
corrupted time values. Also, ensure the doorbell method request is done
while holding the bus vote so the host stays awake and receives the
response as soon as possible.

Change-Id: If876905eb523fd65f3758f13b75c035de4108d8d
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>