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.

Fixes: e9b3e5b8 ("Bluetooth: hci_qca: only assign wakeup with serial port support")
Cc: stable@vger.kernel.org      # 6.2
Cc: Zhengping Jiang <jiangzp@google.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>