drm/bridge: Return NULL instead of plain 0 in drm_dp_hpd_bridge_register() stub

sparse complains:

  drivers/usb/typec/tcpm/qcom/qcom_pmic_typec.c: note: in included file:
  include/drm/bridge/aux-bridge.h:29:16: sparse: sparse: Using plain integer as NULL pointer

Return NULL to clear up the warning.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312060025.BdeqZrWx-lkp@intel.com/
Fixes: e560518a ("drm/bridge: implement generic DP HPD bridge")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231205-drm_aux_bridge-fixes-v1-3-d242a0ae9df4@kernel.org

include/drm/bridge/aux-bridge.h

@@ -26,7 +26,7 @@ void drm_aux_hpd_bridge_notify(struct device *dev, enum drm_connector_status sta
 static inline struct device *drm_dp_hpd_bridge_register(struct device *parent,
 							struct device_node *np)
 {
-	return 0;
+	return NULL;
 }
 
 static inline void drm_aux_hpd_bridge_notify(struct device *dev, enum drm_connector_status status)