fwk: pd: return 0 voltage when no active charge port

For commit c7d2e2c.
When no AC is connected, prev_charge_port is -1. In such cases,
cypd_get_active_port_voltage() should return 0 instead of accessing
pd_port_states[-1], since there is no valid PD voltage without AC.

BRANCH=fwk-main
BUG=PD voltage gets a non 0 value when unplugging the AC
TEST=unplug AC and check the EC log "3lv-buck update! V:0mV"

Signed-off-by: johnwc_yeh <JohnWC_Yeh@compal.com>
(cherry picked from commit 3712efbeae1a3604f02e4aa5e7223f3e3778e52a)

Author: JohnWC-Yeh

zephyr/program/framework/src/cypress_pd_common.c

@@ -1397,6 +1397,9 @@ int cypd_get_ac_power(void)
 
 int cypd_get_active_port_voltage(void)
 {
+	if (prev_charge_port == -1)
+		return 0;
+
 	return pd_port_states[prev_charge_port].voltage;
 }