drivers/isdn/hisax: Drop unused list

The file st5481_init.c locally defines and initializes the adapter_list
variable, but does not use it for anything.  Removing the list makes it
possible to remove the list field from the st5481_adapter data structure.
In the function probe_st5481, it also makes it possible to free the locally
allocated adapter value on an error exit.

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>