Bluetooth: hci_qca: fix NULL pointer dereference in qca_dmp_hdr() for non-serdev...

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

hu->serdev is NULL for hci_uart attached via non-serdev paths, but
qca_dmp_hdr() 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
"hci_ldisc_qca" for the non-serdev case.

Fixes: 06d3fdfcdf5c ("Bluetooth: hci_qca: Add qcom devcoredump support")
Signed-off-by: Zijun Hu <zijun.hu@oss.qualcomm.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>