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

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: e560518a6c2e ("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