samples: modem: hello_hl78xx: select RSSI or RSRP based on RAT mode

The sample previously always queried RSRP, which is not valid when the
HL78xx operates in GSM mode. Use RSSI when GSM RAT is active and RSRP
for other RATs. Update the log output and rename the variable for
clarity.

Signed-off-by: Zafer SEN <zafersn93@gmail.com>

Author: zafersn

samples/drivers/modem/hello_hl78xx/src/main.c

@@ -298,7 +298,7 @@ int main(void)
 	char serial_number[MDM_SERIAL_NUMBER_LENGTH] = {0};
 	enum hl78xx_cell_rat_mode tech;
 	enum cellular_registration_status status;
-	int16_t rsrp;
+	int16_t signal_strength = 0;
 	const char *newapn = "";
 	const char *sample_cmd = "AT";
 
@@ -335,7 +335,11 @@ int main(void)
 	/* Get the current registration status */
 	cellular_get_registration_status(modem, hl78xx_rat_to_access_tech(tech), &status);
 	/* Get the current signal strength */
-	cellular_get_signal(modem, CELLULAR_SIGNAL_RSRP, &rsrp);
+#ifdef CONFIG_MODEM_HL78XX_RAT_GSM
+	cellular_get_signal(modem, CELLULAR_SIGNAL_RSSI, &signal_strength);
+#else
+	cellular_get_signal(modem, CELLULAR_SIGNAL_RSRP, &signal_strength);
+#endif
 	/* Get the current network operator name */
 	hl78xx_get_modem_info(modem, HL78XX_MODEM_INFO_NETWORK_OPERATOR, (char *)operator,
 			      sizeof(operator));
@@ -350,7 +354,11 @@ int main(void)
 	LOG_INF("Imei: %s", imei);
 	LOG_INF("RAT: %s", rat_get_in_string(tech));
 	LOG_INF("Connection status: %s(%d)", reg_status_get_in_string(status), status);
-	LOG_INF("RSRP : %d", rsrp);
+#ifdef CONFIG_MODEM_HL78XX_RAT_GSM
+	LOG_INF("RSSI : %d", signal_strength);
+#else
+	LOG_INF("RSRP : %d", signal_strength);
+#endif
 	LOG_INF("Operator: %s", (strlen(operator) > 0) ? operator : "\"\"");
 	LOG_RAW("**********************************************************\n\n");