Revert "Input: serio - make write method mandatory"

This reverts commit 81c7c0a3. The idea
to make write method mandatory was flawed as several client drivers
(such as atkbd) check for presence of write() method to adjust behavior
of the driver.
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/serio/ams_delta_serio.c

@@ -89,11 +89,6 @@ static irqreturn_t ams_delta_serio_interrupt(int irq, void *dev_id)
 	return IRQ_HANDLED;
 }
 
-static int ams_delta_serio_write(struct serio *serio, u8 data)
-{
-	return -EINVAL;
-}
-
 static int ams_delta_serio_open(struct serio *serio)
 {
 	struct ams_delta_serio *priv = serio->port_data;
@@ -162,7 +157,6 @@ static int ams_delta_serio_init(struct platform_device *pdev)
 	priv->serio = serio;
 
 	serio->id.type = SERIO_8042;
-	serio->write = ams_delta_serio_write;
 	serio->open = ams_delta_serio_open;
 	serio->close = ams_delta_serio_close;
 	strlcpy(serio->name, "AMS DELTA keyboard adapter", sizeof(serio->name));

drivers/input/serio/serio.c

@@ -694,11 +694,6 @@ EXPORT_SYMBOL(serio_reconnect);
  */
 void __serio_register_port(struct serio *serio, struct module *owner)
 {
-	if (!serio->write) {
-		pr_err("%s: refusing to register %s without write method\n",
-		       __func__, serio->name);
-		return;
-	}
 	serio_init_port(serio);
 	serio_queue_event(serio, owner, SERIO_REGISTER_PORT);
 }

include/linux/serio.h

@@ -121,7 +121,10 @@ void serio_unregister_driver(struct serio_driver *drv);
 
 static inline int serio_write(struct serio *serio, unsigned char data)
 {
+	if (serio->write)
 		return serio->write(serio, data);
+	else
+		return -1;
 }
 
 static inline void serio_drv_write_wakeup(struct serio *serio)