drivers/isdn: eliminate duplicated test

The code checked slot_rx twice.  Check slot_tx by analogy with the bank
case.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/

)

// <smpl>
@@
expression E;
@@

(
*E && E
|
*E || E
)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>