Bluetooth: hci_qca: fix a NULL-pointer derefence at shutdown

Unlike qca_regulator_init(), qca_power_shutdown() may be called for
QCA_ROME which does not have qcadev->bt_power assigned. Add a
NULL-pointer check before dereferencing the struct qca_power pointer.

Fixes: eba17187 ("Bluetooth: hci_qca: make pwrseq calls the default if available")
Reported-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Closes: https://lore.kernel.org/linux-bluetooth/su3wp6s44hrxf4ijvsdfzbvv4unu4ycb7kkvwbx6ltdafkldir@4g7ydqm2ap5j/


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>