Fix a prior derp cleanly

src/include/86box/serial.h

@@ -44,8 +44,6 @@
 #define SERIAL4_ADDR		0x02e8
 #define SERIAL4_IRQ		3
 
-#define MAX_SERIAL		4
-
 
 struct serial_device_s;
 struct serial_s;

src/machine/m_pcjr.c

@@ -867,7 +867,7 @@ machine_pcjr_init(const machine_t *model)
     device_add(&fdc_pcjr_device);
 
     device_add(&i8250_pcjr_device);
-    serial_set_next_inst(MAX_SERIAL);	/* So that serial_standalone_init() won't do anything. */
+    serial_set_next_inst(SERIAL_MAX);	/* So that serial_standalone_init() won't do anything. */
 
     return ret;
 }

src/machine/m_xt_zenith.c

@@ -154,7 +154,7 @@ machine_xt_z184_init(const machine_t *model)
     lpt2_remove();
     lpt1_init(0x278);
     device_add(&i8250_device);
-    serial_set_next_inst(MAX_SERIAL);	/* So that serial_standalone_init() won't do anything. */
+    serial_set_next_inst(SERIAL_MAX);	/* So that serial_standalone_init() won't do anything. */
         
     device_add(&cga_device);