drivers: wifi: Fix memory leak in regulatory processing

During Wi-Fi connection UMAC sends an unsolicited regulatory change
event but the driver code always assumes that this event is solicited
hence doesn't free the memory for the event.

Fix this by dropping the unsolicited event as it's not supported yet.

Fixes #79733.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>