Allow the use of a single IS31FL3731 LED driver

quantum/rgb_matrix_drivers.c

@@ -31,7 +31,9 @@ static void init(void) {
     i2c_init();
 #    ifdef IS31FL3731
     IS31FL3731_init(DRIVER_ADDR_1);
+#      ifdef DRIVER_ADDR_2
     IS31FL3731_init(DRIVER_ADDR_2);
+#      endif
 #    elif defined(IS31FL3733)
     IS31FL3733_init(DRIVER_ADDR_1, 0);
 #    elif defined(IS31FL3737)
@@ -55,7 +57,9 @@ static void init(void) {
     // This actually updates the LED drivers
 #    ifdef IS31FL3731
     IS31FL3731_update_led_control_registers(DRIVER_ADDR_1, 0);
+#      ifdef DRIVER_ADDR_2
     IS31FL3731_update_led_control_registers(DRIVER_ADDR_2, 1);
+#      endif
 #    elif defined(IS31FL3733)
     IS31FL3733_update_led_control_registers(DRIVER_ADDR_1, 0);
     IS31FL3733_update_led_control_registers(DRIVER_ADDR_2, 1);
@@ -69,7 +73,9 @@ static void init(void) {
 #    ifdef IS31FL3731
 static void flush(void) {
     IS31FL3731_update_pwm_buffers(DRIVER_ADDR_1, 0);
+#      ifdef DRIVER_ADDR_2
     IS31FL3731_update_pwm_buffers(DRIVER_ADDR_2, 1);
+#      endif
 }
 
 const rgb_matrix_driver_t rgb_matrix_driver = {