CVE-2026-74304: Bluetooth: hci_qca: fix NULL pointer dereference in qca_setup() for non-serdev device

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: hciqca: fix NULL pointer dereference in qcasetup() for non-serdev device

hu->serdev is NULL for hciuart attached via non-serdev paths, but qcasetup() unconditionally calls serdevdevicegetdrvdata(hu->serdev) and dereferences the result, causing a NULL pointer dereference.

Fix by guarding the dereference with a NULL check, consistent with the rest of qcasetup().