slimbus: qcom-ngd-ctrl: Avoid accessing deallocated stack

The functions qcom_slim_ngd_xfer_msg and
qcom_slim_ngd_xfer_msg_sync declare a local completion
variable called done. However, this variable is accessed
beyond the scope of these functions.

To address this issue:
1. Instead of keeping done as a local variable,
move it to qcom_slim_ngd_ctrl.
2. Initialize done during the probe phase.
3. Use this variable for handling transfer and
synchronization messages.

Change-Id: If97b71e2db730ab21bfd07479d2737b0546e1f8e
Signed-off-by: Mehul Raninga <quic_mraninga@quicinc.com>