hysdn: fix writing outside the field on 64 bits

ifa_local is assumed to be unsigned long which lead to writing the address
at dev->dev_addr-2 instead of +2

noticed thanks to gcc:

drivers/isdn/hysdn/hysdn_net.c: In function `net_open':
drivers/isdn/hysdn/hysdn_net.c:91: warning: array subscript is below array bounds

Signed-off-by: Pascal Terjan <pterjan@mandriva.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>