CVE-2026-74303: Bluetooth: hci_qca: fix NULL pointer dereference in qca_dmp_hdr() for non-serdev device

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

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

hu->serdev is NULL for hciuart attached via non-serdev paths, but qcadmphdr() unconditionally dereferences hu->serdev->dev.driver->name, causing a NULL pointer dereference.

Fix by guarding the dereference with a NULL check and falling back to "hcildiscqca" for the non-serdev case.