staging: rtl8712: style fix returned error code

This patch fixes a coding style issues in the rtl8712 module as noted
by checkpatch.pl regarding the returned error code.

It fixes the following checkpatch.pl warning:

WARNING: ENOSYS means 'invalid syscall nr' and nothing else

Signed-off-by: Martin Homuth <martin@martinhomuth.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8712/rtl871x_ioctl_linux.c

@@ -1853,7 +1853,7 @@ static int dummy(struct net_device *dev,
 		struct iw_request_info *a,
 		union iwreq_data *wrqu, char *b)
 {
-	return -ENOSYS;
+	return -EINVAL;
 }
 
 static int r8711_drvext_hdl(struct net_device *dev,