isdn: eicon: Use offsetof

Use offsetof instead of explicit implementation.

* fixes bug with omitted & like:
  len = (byte)(((T30_INFO *) 0)->station_id + 20)

* avoids compiler warnings with wrong sizes (pointer-to-char cast):
  len = (byte)(&(((T30_INFO *) 0)->universal_6));

* cleans up the code

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>