isdn: eicon: Return on error

When diva_strace_read_uint returns an error, return even from
process_idi_event, because l2_state is uninitialized.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Karsten Keil <isdn@linux-pingi.de>
Acked-by: Armin Schindler <armin@melware.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>