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.

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>