staging: comedi: return error on "write" if no command set up

The "write" file operation handler, `comedi_write()` returns an error
for pretty much any condition that prevents a "write" going ahead.  One
of the conditions that prevents a "write" going ahead is that no
asynchronous command has been set up, but that currently results in a
return value of 0 (unless COMEDI instructions are being processed or an
asynchronous command has been set up by a different file object).
Change it to return `-EINVAL` in this case.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -2331,9 +2331,12 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 	}
 
 	async = s->async;
-
-	if (!s->busy || !nbytes)
+	if (!nbytes)
+		goto out;
+	if (!s->busy) {
+		retval = -EINVAL;
 		goto out;
+	}
 	if (s->busy != file) {
 		retval = -EACCES;
 		goto out;
@@ -2373,8 +2376,10 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 				retval = -ERESTARTSYS;
 				break;
 			}
-			if (!s->busy)
+			if (!s->busy) {
+				retval = -EINVAL;
 				break;
+			}
 			if (s->busy != file) {
 				retval = -EACCES;
 				break;