usb: typec: ucsi_glink: Fix a NULL pointer dereference

During a SSR/PDR scenario, when pmic_glink state is down,
ucsi_glink driver unregisters from the UCSI framework thereby
setting udev->ucsi to NULL. Typically, we don't expect any
notification coming from the charger firmware that runs on a
remote subsystem until pmic_glink state is up and ucsi_glink
driver registers again with the UCSI framework. In the case,
if ucsi_glink driver receives a notification before UCSI driver
finishes the registration, then don't handle it.

Change-Id: Ieb792ca6026344462c7eae4d40aef87ae061ae96
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>