tcpci: anx7688: enable message passing after hard reset sent

In TCPCI specifiction R1.0 4.7.2, the last step of transmitting
hard reset message is enable PD message passing by writing to
RECEIVE_MESSAGE register.

BRANCH=none
BUG=chrome-os-partner:52815
TEST=manual
  build and load on reference board with anx7688 port controller.
  connect zinger to port 0, and use ec uart console to send hard
  reset message:
    pd 0 hard
  check PD communication

Signed-off-by: Rong Chang <rongchang@chromium.org>
Change-Id: I52968b603f0227d7d9a112b0216cd5fd6362a0b2
Reviewed-on: https://chromium-review.googlesource.com/348142
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Koro Chen <koro.chen@mediatek.com>
Reviewed-by: Wei-Ning Huang <wnhuang@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>

common/usb_pd_protocol.c

@@ -2135,6 +2135,16 @@ void pd_task(void)
 			}
 			if (pd_is_vbus_present(port) &&
 			    snk_hard_reset_vbus_off) {
+#ifdef CONFIG_USB_PD_TCPM_TCPCI
+				/*
+				 * After transmitting hard reset, TCPM writes
+				 * to RECEIVE_MESSAGE register to enable
+				 * PD message passing.
+				 */
+				if (pd_comm_enabled)
+					tcpm_set_rx_enable(port, 1);
+#endif /* CONFIG_USB_PD_TCPM_TCPCI */
+
 				/* VBUS went high again */
 				set_state(port, PD_STATE_SNK_DISCOVERY);
 				timeout = 10*MSEC;