x25_asy: Fix ref count rule violation

x25_asy does not take an ldisc reference before calling the flush method.
Fix it to use the helper function we provide.

Signed-off-by: Alan Cox <alan@redhat.com>
Cc: Krzysztof Halasa <khc@pm.waw.pl>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>