tty: serdev: fix serdev_device_write return value

Commit 6fe729c ("serdev: Add serdev_device_write subroutine")
provides a compatibility wrapper for the existing
serdev_device_write_buf, but it fails to return the number of bytes
written causing users to timeout.

Fixes: 6fe729c ("serdev: Add serdev_device_write subroutine")
Cc: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Author: robherring

drivers/tty/serdev/core.c

@@ -139,7 +139,7 @@ int serdev_device_write(struct serdev_device *serdev,
 			unsigned long timeout)
 {
 	struct serdev_controller *ctrl = serdev->ctrl;
-	int ret;
+	int ret, wr_cnt = 0;
 
 	if (!ctrl || !ctrl->ops->write_buf ||
 	    (timeout && !serdev->ops->write_wakeup))
@@ -155,12 +155,13 @@ int serdev_device_write(struct serdev_device *serdev,
 
 		buf += ret;
 		count -= ret;
+		wr_cnt += ret;
 
 	} while (count &&
 		 (timeout = wait_for_completion_timeout(&serdev->write_comp,
 							timeout)));
 	mutex_unlock(&serdev->write_lock);
-	return ret < 0 ? ret : (count ? -ETIMEDOUT : 0);
+	return ret < 0 ? ret : (count ? -ETIMEDOUT : wr_cnt);
 }
 EXPORT_SYMBOL_GPL(serdev_device_write);