AP_ADSB: use have_serial when detecting

avoid find_serial() as it changes port options

libraries/AP_ADSB/AP_ADSB_Sagetech.cpp

@@ -45,7 +45,7 @@
 // detect if any port is configured as Sagetech
 bool AP_ADSB_Sagetech::detect()
 {
-    return (AP::serialmanager().find_serial(AP_SerialManager::SerialProtocol_ADSB, 0) != nullptr);
+    return AP::serialmanager().have_serial(AP_SerialManager::SerialProtocol_ADSB, 0);
 }
 
 // Init, called once after class is constructed

libraries/AP_ADSB/AP_ADSB_uAvionix_UCP.cpp

@@ -45,7 +45,7 @@ extern const AP_HAL::HAL &hal;
 // detect if any port is configured as uAvionix_UCP
 bool AP_ADSB_uAvionix_UCP::detect()
 {
-    return (AP::serialmanager().find_serial(AP_SerialManager::SerialProtocol_ADSB, 0) != nullptr);
+    return AP::serialmanager().have_serial(AP_SerialManager::SerialProtocol_ADSB, 0);
 }