CVE-2024-35850 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.