usb: dwc3-msm: Use dummy buffer as doorbell until GSI is ready

USB GSI wrapper can try to access IPA GSI Doorbell as soon as
endpoint is enabled and transfers are started. However there
is always going to be some delay when transfers are started
and GSI doorbell address is updated. This may result in USB
GSI wrapper accessing address-0. Prevent this by using some
dummy buffer as doorbell until IPA GSI connections are setup.
At that stage driver will override doorbell address with valid
one.

Change-Id: I477602d33737a994992c5394c6c08d63744e525f
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>