AP_SerialManager: fixed GPS in AP_Periph

we need to have at least 4 SERIALn_* parameters to support GPS on
AP_Periph due to the odd ordering of hal.uartB as SERIAL3

libraries/AP_SerialManager/AP_SerialManager.h

@@ -26,8 +26,14 @@
 #include <AP_Param/AP_Param.h>
 
 #ifdef HAL_UART_NUM_SERIAL_PORTS
+#if HAL_UART_NUM_SERIAL_PORTS >= 4
 #define SERIALMANAGER_NUM_PORTS HAL_UART_NUM_SERIAL_PORTS
 #else
+// we need a minimum of 4 to allow for a GPS due to the odd ordering
+// of hal.uartB as SERIAL3
+#define SERIALMANAGER_NUM_PORTS 4
+#endif
+#else
 // assume max 8 ports
 #define SERIALMANAGER_NUM_PORTS 8
 #endif