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

The "read" file operation handler, `comedi_read()` returns an error for
pretty much any condition that prevents a "read" going ahead.  One of
the conditions that prevents a "read" 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

@@ -2478,8 +2478,12 @@ static ssize_t comedi_read(struct file *file, char __user *buf, size_t nbytes,
 	}
 
 	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;
@@ -2517,6 +2521,7 @@ static ssize_t comedi_read(struct file *file, char __user *buf, size_t nbytes,
 				break;
 			}
 			if (!s->busy) {
+				retval = -EINVAL;
 				break;
 			}
 			if (s->busy != file) {