Bluetooth: qca: fix NULL-deref on non-serdev setup

In the Linux kernel, the following vulnerability has been resolved: Bluetooth: qca: fix NULL-deref on non-serdev setup Qualcomm ROME controllers can be registered from the Bluetooth line discipline and in this case the HCI UART serdev pointer is NULL. Add the missing sanity check to prevent a NULL-pointer dereference when setup() is called for a non-serdev controller.