msm: gsi: allow start_xfer when channel is not running

GSI doorbell can be rang even if the channel is not running.
The doorbell will be cached and will be applied when
channel moves to running state. This change allow ringing
the doorbell for channels before starting them to avoid a race
condition between initial buffer replenish and channel start.

Change-Id: Iea187b98319c16a259d7b8ba5ee9abc338f52c30
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>