init: Initialize serial subsystem before init_earlyinitcalls()

Otherwise, the serial sybsystem isn't initialized before the serial
drivers attempt to register themselves.

drivers/serial.c

@@ -33,8 +33,12 @@ struct serial_dev *serial_first_device() {
 	return first_serial_dev;
 }
 
+/*
+ * Initialize the serial sybsystem.
+ * Note: this is called directly from the kernel initialization as opposed to
+ * being an initcall because it has to be called before the serial drivers get
+ * initialized in earlyinitcalls.
+ */
 void serial_init() {
 	first_serial_dev = 0;
 }
-
-driversubsys_initcall(serial_init);